🛠️ For the DM

Running Your First Session: A First-Time DM's Guide

A practical guide for the first-time DM

You are about to do something most people only ever dream about: you're going to build a world, breathe life into it, and hand five people an experience they'll quote at each other for years. The setting is the grimdark 41st Millennium — cathedral spaceships, chainswords, the Emperor rotting on His Golden Throne — and your players are going to love it because you love it.

You don't need to know every rule. You don't need a perfect script. You need a handful of scenes, a gut instinct for when things feel right, and the confidence to say "yes, and…" You have all of that. This guide is just here to remind you of it.

The One Mindset That Changes Everything

You are the players' biggest fan.

Read that again. Your job is not to challenge them. It's not to protect the story you planned. It's not to be a neutral referee. It is to want their crazy ideas to work — and to let them work whenever you possibly can.

When a player says "Can I grab the Chaos Marine's helmet mid-combat and use it as a weapon?" the answer from the rules-lawyer DM is "that's not in the PHB." The answer from the players' biggest fan is: "Yes — make an Athletics check. If you beat DC 14, you've got a jury-rigged improvised weapon dealing 1d4 bludgeoning and the Marine looks ridiculous." They'll be talking about that moment for months.

Every encounter, every scene, every ruling: ask yourself "How do I make this work for them?" That question will carry you through every situation this guide doesn't cover.

Prep Situations, Not Scripts

This is the single most important lesson, and it is going to save you hours of wasted prep.

Your players will not do what your script assumes. If you write: "The players enter the hive-city slum, follow the cultist to the warehouse, and confront the Chaos sorcerer" — your players will immediately try to arrest the cultist, or set the warehouse on fire from outside, or attempt to talk to the sorcerer about his feelings. The script dies in the first five minutes, and suddenly you're improvising anyway, except now you're also stressed about all that prep you can't use.

Instead, prep a situation:

A location with a feel (crumbling Administratum annex, stinking forge-pit, a shrine to the Emperor with fresh blood on the floor)
NPCs who want something (the cult preacher wants to finish the summoning ritual; the captured Guardsman wants to survive; the Inquisitor's informant wants out of this life)
A problem (a daemon is being summoned in an hour — the clock is ticking)
A few clues and leads the players can pick up and follow in whatever order they choose

Know what each NPC wants and how they'd react to being bribed, threatened, ignored, or killed. Then let the scene run. You're not writing what will happen — you're building a world that would react to whatever they try.

For this one-shot, prep 3–5 set pieces loosely linked:

An opening scene (in media res — see below)
An exploration or social scene (interrogate the informant, find the shrine)
A mid-fight (cultists + a mini-boss)
A puzzle or hazard (a warp-instability zone, a ticking bomb, an unstable bulkhead)
A climax (the big fight or confrontation)

Have 1–2 extra random encounters ready that you never have to use. Have a couple of spare stat blocks. Then close the notebook and trust yourself.

Start Strong — In Media Res

Do not open in a tavern. Do not open with "you all meet at the checkpoint." Do not start with five minutes of world-building.

Drop them into a moment.

"The Valkyrie's ramp is already down and you're already falling — 40 feet above the hive-city slum, the jump harness screaming in your ear. Below you: a warehouse, a dozen robed figures, and something that is not a fire burning in the center of the floor. You hit the roof in three seconds. What do you do?"

"The Commissar is already dead. You saw it happen — the cultist's blade, the hiss of a warp-curse, and then silence. His power sword is still in his hand. The cell door is locked. There are footsteps outside. You have about thirty seconds. Go."

That's your opening. One scene, maximum tension, immediate choice. They are already a party — pre-gen characters should have a one-line bond to each other ("you served together in the 23rd Cadian") and a one-line hook to the mission ("the Inquisitor has evidence your brother is a heretic — you need to see it first"). Handle the backstory in play, not in a preamble.

When to Call for a Roll — and When to Just Say Yes

This is where a lot of first-time DMs tie themselves in knots. Here's the rule:

Call for a check only when (1) the outcome is uncertain AND (2) failure is interesting.

If a trained soldier wants to climb a 10-foot ladder — that's not uncertain. Narrate it and move on.

If the player says something genuinely clever or persuasive in-character, and you're about to say "okay, roll Persuasion" — stop. Ask yourself: if they rolled a 2, would I actually say it didn't work? If the answer is no, don't roll. Give it to them. The dice exist to introduce drama and uncertainty, not to randomly invalidate good play.

Decide the consequence of failure before you call for the roll. If you can't name an interesting failure state — something that makes the story more complicated, not just "nothing happens" — don't ask for a roll.

Good reasons to roll:

There's a real chance of failure that would be interesting ("you might set off the alarm, which gives the cult 2 rounds to prepare")
A player is attempting something risky against resistance ("the Guardsman is trying to hold the blast door against a Chaos Marine")
Time or resources are at stake ("can you rewire the servo-skull before the patrol reaches you?")

Bad reasons to roll:

You want to check if the players "notice" the only clue that unlocks the next scene (if they don't find it, the game grinds to a halt — just give them the clue)
You want to see if a character "knows" lore that the player already figured out
You're uncomfortable saying yes

Setting DCs — Fast and Confident

Use the ladder. Commit to it before the dice hit the table, never after.

Difficulty	DC	The vibe
Very Easy	5	A child could do it; only fail if rattled
Easy	10	Routine for a trained person
Medium	15	Your default when unsure
Hard	20	Push the limits of human ability
Very Hard	25	Legendary feat
Nearly Impossible	30	Defying mortal limits

When in doubt: DC 15. It's in the middle; it respects both the possibility of success and failure; it's where most tense moments live. Pick it, say it out loud ("give me a DC 15 Persuasion check"), and let the dice fall.

Never retroactively raise a DC because the players succeeded too easily. Never lower it because you feel bad. You called it — honor it. Adjust your expectations for next time instead.

One bonus tool: advantage instead of a lower DC is the best way to reward clever play. A player who positions the Servo-skull as a distraction before sneaking past the guard doesn't get DC 10 — they still get DC 15, but with advantage. That's your "yes, and" mechanic built right into the dice.

Running a Clean Combat Round

Combat has a reputation for bogging down. The fix is pacing: keep the loop tight.

Before the fight starts: build an initiative list on your DM screen or a notepad — players and monsters both. Keep it visible. Call names in order; move fast.

Each turn, in order:

"It's [Name]'s turn." (If they need a second, immediately: "And [Next Name], you're on deck, get ready.")
Player announces what they're doing.
Resolve it — ask for the roll if needed.
Narrate the result with color, not just numbers. Don't say "17 hits, 8 damage." Say: "Your lasgun blast punches through the cultist's robe and he drops, clutching at the burning hole in his chest." Twelve more words, ten times more memorable.
Move to the next turn immediately.

Monster turns: you don't need elaborate tactics on your first run. Move toward the nearest target, attack, maybe take cover if it makes sense. The goal isn't to optimize the enemy — it's to create a threatening scene. Describe monster attacks with the same color: "The Ork Nob roars 'WAAAGH!' and brings his choppa down in a wild overhead swing — that's a 14 against your AC."

Roll monster attacks openly (or at least look like you are). Players trust a table where the dice are out in the open. A lucky monster crit is exciting; a secret DM fudge is flat.

The golden rule of combat pacing: if a player is spending more than 60 seconds deciding what to do, gently say "What does [Character] do in the chaos of the moment?" — it helps them commit. Never let one player's deep optimization hold five other people hostage.

Sharing the Spotlight

A good one-shot gives every player a moment that's theirs. Watch for who's been quiet.

Deliberately build scenes that hand the ball to different people:

The lock or the warp-tech terminal (the rogue's moment, or the Admech character's)
The Imperial officer or local Arbites commander the party needs to talk past (the Rogue Trader or the Commissar-type's moment)
The heavy-bolter position covering the retreat (the Guardsman fighter's moment)
The psyker sign on the wall (the Psyker's moment to read the residue)

If you realize the same player has carried the last three scenes, find a moment to pivot: "Great — while [Dominant Player] does that, what's [Quiet Player] watching on the perimeter?" Give them the ball. They might surprise you.

After the climax, give every character one beat — one line of resolution or reaction. Even just: "[Name], what does your character do in the silence after the daemon falls?" That moment costs ten seconds and people remember it.

"Yes, And…" / "Yes, But…"

These are the two moves that will make you feel like you know what you're doing.

"Yes, and…" — you succeed, and something extra happens: "Yes, you convince the sergeant to let you through the checkpoint — and he offers to radio ahead to the armoury, which means you might be able to pick up a medi-pack."

"Yes, but…" — you succeed, with a complication or cost: "Yes, you manage to hack the cogitator and pull the manifest — but the door alarm trips as you do it. You've got 30 seconds before someone comes to investigate."

These phrases almost never require a rule. They keep the story moving forward, they honor player effort, and they generate drama automatically by adding a wrinkle even on a success. Use them constantly.

"No, but…" is your third tool for when something genuinely can't work: "There's no way to sneak past the Chaos Marine — he's at a choke point with a direct sightline — but there's a maintenance hatch on your right that bypasses the corridor entirely." You've said no, but you've given them a path. The world resists; it doesn't wall them in.

Handling a Rule You Don't Know

You will get asked a rules question you can't answer. It will happen in the first hour. Here is exactly what to do:

Make a fast, fair ruling. Say it out loud. Move on.

"I think in this case let's say it's a DC 13 Dexterity save — if you fail, you take the blast. We'll check the actual rule at the break."

A confident wrong ruling that keeps momentum going is better than a five-minute rulebook dive every time. Your players didn't come here to watch you read the PHB. They came to play. The ruling doesn't need to be right — it needs to be reasonable and decisive.

Note the question on a scrap of paper. Look it up at the next break. If your ruling was wrong, acknowledge it and say how you'd handle it next time. Nobody will care. They'll be too busy reliving the moment your wrong ruling enabled.

The only rules you need cold (they come up constantly): the conditions (see the reference card), the action options, advantage/disadvantage, and how death saves work. Everything else can wait for a break.

Pacing a 3–4 Hour One-Shot

The clock is the one thing that will genuinely hurt you if you ignore it. A one-shot must reach its ending. There is no "next session" to wrap things up.

Rough time budget for a 4-hour block:

Phase	Time
Intro, safety tools, character bonds	15 min
Opening scene / hooks / first exploration	40–45 min
Mid-section (a fight + a social or puzzle beat)	50–60 min
Break	10 min
Rising tension / second fight or complication	40–45 min
Climax (the big final scene)	40–45 min
Denouement — player beats, loose ends	10–15 min

Check the clock at the break. If you're behind, cut one of the mid-section scenes — drop the extra fight or shorten the exploration. Protecting the climax is more important than including everything you prepped.

Signs you need to accelerate:

A scene has been running 30+ minutes with no decision point in sight
Players are going in circles trying to find information (give them a clue for free)
The second fight is looking like it'll eat 45+ minutes

Hard cut if needed: "You've dealt with this — as you move out, a distant explosion rocks the building and you realize the ritual is nearly complete. You don't have time for anything else. Run." That kind of forced push to the climax feels like drama, not a railroad. Use it.

Reading the Table

Watch the room. It tells you more than the dice.

You're losing them when:

Phones come out
Side conversations start
Players slump or start asking unrelated questions
The current scene has no obvious decision to make

Fix: cut to an action beat, hand someone a decision, introduce a new threat or revelation. Accelerate. Ask a direct question to a specific player: "[Name], your character has a gut feeling about this — what does it tell you?"

You've got them when:

Players argue in-character about what to do
They lean forward
Someone laughs, then immediately gets back in it
They start quoting the setting back at you ("By the Emperor, he's already corrupted")

Fix: nothing. Let it ride. This is the magic. Resist the urge to introduce the next thing before this thing is done.

Ending Strong

Land the climax. Don't rush past it with "and then the daemon is defeated." Give it weight.

After the final fight or confrontation, take a breath (real, actual pause), then go around the table:

"[Name] — in the silence after, what does your character do or say?"

One at a time. Let them each have a moment. It takes three minutes total and it is the thing they will remember most.

Then close with a single line of consequence:

"As you report to the Inquisitor, she reads your account without expression. 'The ritual failed,' she says, 'this time.' She hands you each a sealed missive. 'Get some rest.' She doesn't look up when you leave. Somewhere above you, in orbit, the Hammer of Righteousness fires its engines. Somewhere below you, in the catacombs, something that survived the purge opens its eyes."

They don't need answers. They need the feeling that the world is larger than one evening.

Phrases You Can Steal at the Table

These are field-tested. Say them. They work.

To build momentum:

"What does [Name] do in the heat of the moment?" (great for hesitant players)
"You've got about 30 seconds before that changes — go."
"The Emperor's eyes are on you right now. What's it going to be?"

To buy yourself time when you're uncertain:

"Interesting — give me a moment to see how the world reacts to that."
"I'm going to rule it works this way for now; we'll check that at the break."
"Tell me what you're trying to accomplish, not just what you're doing." (clarifies intent before you rule)

To reward clever thinking:

"That's exactly the kind of thinking that keeps soldiers alive. Roll with advantage."
"That's genuinely smart — I'll say the DC drops to 12 for that approach."

To keep the spotlight moving:

"Great. While [Name] handles that — [Other Name], where are you in all this?"
"[Quiet player], your character's been watching this whole thing unfold. What's your read?"

To raise the stakes:

"You can do it — but it's going to cost you. What are you willing to spend?"
"That works. But something is watching you from the shadows, and it knows it works too."

To handle a player going off-script:

"Yes — and here's how the world responds to that."
"I love it. Here's what that costs / what it opens up."

In the 40k register specifically:

"The light of the Astronomican flickers for just a moment — everyone roll Wisdom." (pure atmosphere, costs nothing)
"The vox crackles. You can't make out words. That's not static."
"Suffer not the witch to live. That's what the Creed says. What do you do?"

Common First-Timer Traps (and How to Dodge Them)

Trap	What it looks like	The dodge
The railroad	Players find the only correct path leads to your planned scene	Prep situations with multiple solutions; honor any reasonable path to the outcome
Death by dice	Calling for a roll every time someone tries anything	Only roll when failure is interesting AND uncertain — say yes freely
Fudging visibly	Checking your notes, then the monster "misses" on a 19	Roll openly; let crits land. Players survive hits. Predictable dice = trust
Forgetting NPC motivations	Every NPC acts as a passive obstacle	Know what each NPC wants; they pursue it whether or not the party notices
Combat crawl	One player takes 5 minutes per turn; combat takes 2 hours	"You're on deck" prep; move to the next initiative; keep narration short and vivid
Lore-dump opening	10-minute explanation of the Imperium before anyone rolls	Reveal lore in play through what they see, hear, and smell — trust the fiction
Running out of time	The finale never happens because the middle ran long	Check the clock at the break; cut a mid-scene; protect the climax ruthlessly
Saying no to creativity	"You can't do that, it's not in the rules"	"Yes, but here's what it costs / here's the DC" — almost always better
Punishing the plan	Players make a smart plan; you make it fail anyway	Let clever plans work. Complications come from the world, not from you wanting them to
Forgetting quiet players	One player carries every scene	Name them, give them a moment, physically direct a question at them

One Last Thing

You have watched the lore. You know the setting. You care enough about your players to read a guide like this one, which means you already have the most important qualification a first-time DM can have.

The grimdark 41st Millennium is a beautiful backdrop for a story — huge, dark, operatic, and just absurd enough to laugh at when the dice go sideways. Your players are going to feel the weight of a universe that doesn't care about them, and they're going to push back anyway, because that's what players do.

When it's over — when the daemon is banished or the ritual fails or the Chaos Marine finally goes down — and everyone's talking over each other about that moment in round three when it all could have fallen apart, you're going to realize that the rules were never the point. You were the point. The shared space you held for them for four hours was the point.

The Emperor protects.

And so do you.

Part of the Warhammer 40,000 One-Shot DM Kit · D&D 5e rules (RAW) with 40k reskin · For your table