🛠️ For the DM
DM SCREEN — At-a-Glance Rules Reference
"In the grim darkness of the far future, there is only war." — and a fistful of d20s.
For your table — D&D 5e (RAW) with a Warhammer 40,000 reskin.
This is your behind-the-screen cheat sheet. Every rule here is straight 5e by the book — only the examples wear 40k armor. When a player asks "can I do that?", the answer is on this page. When you don't know a rule, make a fast call out loud and look it up at the break (see the note at the bottom). Keep the game moving.
ABILITY CHECK DC LADDER
Set the DC before the roll, not after. When you're not sure, default to DC 15 (Medium). The roll is: d20 + ability modifier + proficiency bonus (if proficient) vs. the DC.
| Difficulty | DC | 40k example |
|---|---|---|
| Very Easy | 5 | Kick open a rotted bulkhead door; recall that "the Emperor protects" |
| Easy | 10 | Climb a cargo gantry; hear a Grot skittering in the vents |
| Medium | 15 | Hotwire a corrupted cogitator; talk past a suspicious Arbites checkpoint |
| Hard | 20 | Decrypt a heretic data-slate; leap a chasm in full flak armor |
| Very Hard | 25 | Out-debate an Inquisitor; repair a machine-spirit mid-firefight |
| Nearly Impossible | 30 | Resist a daemon's whispered bargain; pilot a crashing lander to a safe set-down |
Only roll when (1) the outcome is uncertain AND (2) failure is interesting. If success is automatic, or a miss just stalls the game, narrate it — don't roll. Decide the consequence of failure before you ask for the dice.
SKILLS & GOVERNING ABILITY
Each skill is tied to one ability. A character adds their proficiency bonus only if trained in that skill. (40k reskins in parentheses — flavor only.)
| Ability | Skills |
|---|---|
| Strength (STR) | Athletics (breaching, hauling wargear, melee shoving) |
| Dexterity (DEX) | Acrobatics, Sleight of Hand, Stealth (infiltration, void-walking) |
| Constitution (CON) | (no skills — used for saves, concentration, gut endurance) |
| Intelligence (INT) | Arcana (Warpcraft/Daemonology), History (Imperial Creed & lore), Investigation, Nature (xenobiology), Religion (the Imperial Cult) |
| Wisdom (WIS) | Animal Handling (cyber-mastiffs, Kroot hounds), Insight, Medicine (Apothecary/narthecium), Perception, Survival (deathworld tracking) |
| Charisma (CHA) | Deception, Intimidation, Performance, Persuasion |
House add (optional, for Tech-Priests): treat Tech and Science as INT skills. Drop them if you want to keep it pure RAW. Passive checks: a passive score =
10 + all modifiers(no roll). Passive Perception is the big one — it sets what a character notices without looking (e.g. spotting a lurking Genestealer).
ACTION ECONOMY (per turn)
What every creature gets on its turn. The most common mistake is treating Bonus Actions as free — they're not.
| Slot | How many | Rule |
|---|---|---|
| Action | 1 | The big thing: Attack, Cast a Spell, Dash, etc. (full list below) |
| Bonus Action | 1 | Only if a feature/spell explicitly grants one. Not free-floating — if nothing gives you a bonus action, you don't have one |
| Move | up to speed | Spend up to your speed; can be split before/after your action. Difficult terrain costs 2 ft per 1 ft |
| Reaction | 1 per round | Off-turn trigger: opportunity attack, Shield, a Readied action. Refreshes at the start of your turn |
| Free action | a few | A few words of speech; communicating briefly |
| Object interaction | 1 free / turn | Draw or stow a weapon, open an unlocked door, pull a lever, grab a stim off your belt. A second interaction costs your whole Action |
Opportunity attack: when a hostile creature leaves your reach (moves out of 5 ft) without Disengaging, you may use your reaction to make one melee attack. Standing up from prone, and movement that doesn't leave your reach, don't provoke.
STANDARD COMBAT ACTIONS
Pick one as your Action each turn (some features grant extras).
| Action | One-line rule |
|---|---|
| Attack | One melee or ranged attack. Extra Attack feature = more swings with this one action |
| Cast a Spell | Cast a spell with a casting time of 1 action (some take longer — check the spell) |
| Dash | Gain extra movement equal to your speed this turn (double your move) |
| Disengage | Your movement this turn doesn't provoke opportunity attacks |
| Dodge | Attacks against you have disadvantage; you have advantage on DEX saves — until your next turn. (Ends if incapacitated or speed 0) |
| Help | Give an ally advantage on an ability check, OR on their next attack vs. a creature within 5 ft of you (before your next turn) |
| Hide | Make a Stealth check to become unseen/unheard. Need cover or obscurement; you can't hide from something that sees you clearly |
| Ready | Pick a trigger + a response now ("when the cultist steps through, I fire"). Uses your reaction when it triggers. Readying a spell costs the slot now and holds concentration |
| Search | Devote your attention to finding something — Perception (spot) or Investigation (deduce), DM's call |
| Use an Object | Interact with an object that needs your full action (a second object interaction, or a complex device) |
| Grapple | (Special melee attack — part of the Attack action.) Your Athletics vs. target's Athletics or Acrobatics (they choose). Success → Grappled (speed 0). Only on creatures up to one size larger |
| Shove | (Special melee attack — part of the Attack action.) Same contest as Grapple. Success → knock the target Prone OR push it 5 ft away |
COVER
Cover gives a bonus to AC and Dexterity saving throws. Use it constantly — 40k is a war of trenches, rubble, and rockcrete.
| Cover | Bonus | Example |
|---|---|---|
| Half cover | +2 AC & DEX saves | A low barricade, a corpse-pile, a hab-block windowsill |
| Three-quarters cover | +5 AC & DEX saves | An arrow-slit, a gun-port, peeking around a Rhino's hull |
| Total cover | Can't be targeted directly | Fully behind a bulkhead — though area effects may still reach |
CONDITIONS (full list)
The effects a creature can suffer. One line each — these come up constantly.
| Condition | Effect |
|---|---|
| Blinded | Can't see; auto-fails sight-based checks. Attacks against it have advantage; its attacks have disadvantage |
| Charmed | Can't attack the charmer or target it with harmful effects. Charmer has advantage on social checks with it |
| Deafened | Can't hear; auto-fails hearing-based checks |
| Frightened | Disadvantage on checks & attacks while the source is in line of sight. Can't willingly move closer to the source |
| Grappled | Speed 0, no bonus to speed. Ends if the grappler is incapacitated or the target is moved out of reach |
| Incapacitated | No actions and no reactions |
| Invisible | Can't be seen without special senses (heavily obscured for hiding). Attacks against it have disadvantage; its attacks have advantage |
| Paralyzed | Incapacitated; can't move or speak; auto-fails STR & DEX saves. Attacks against it have advantage; any hit from within 5 ft is a critical |
| Petrified | Turned to inanimate stone: incapacitated, unaware, can't move/speak; weight ×10; resistance to all damage; immune to poison & disease (existing ones suspended); auto-fails STR & DEX saves; attacks against it have advantage |
| Poisoned | Disadvantage on attack rolls and ability checks |
| Prone | Can only crawl (or stand, costing half your speed). Disadvantage on attacks. Melee attacks against it have advantage; ranged attacks against it have disadvantage |
| Restrained | Speed 0. Attacks against it have advantage; its attacks have disadvantage; disadvantage on DEX saves |
| Stunned | Incapacitated; can't move; can only falter out a few words; auto-fails STR & DEX saves. Attacks against it have advantage |
| Unconscious | Incapacitated, prone, drops what it holds, unaware. Auto-fails STR & DEX saves; attacks against it have advantage; any hit from within 5 ft is a critical |
Exhaustion (6 levels — cumulative; each level adds to the lower ones)
A failed forced-march, no rest in the warp-haunted dark, a sucking chest wound that won't heal.
| Level | Effect |
|---|---|
| 1 | Disadvantage on ability checks |
| 2 | Speed halved |
| 3 | Disadvantage on attack rolls and saving throws |
| 4 | HP maximum halved |
| 5 | Speed reduced to 0 |
| 6 | Death |
A long rest removes one level of exhaustion (if the creature has also had food and drink).
DEATH SAVES
At 0 HP a creature is unconscious and dying. On each of its turns, roll a d20 — no modifiers (no proficiency, no ability mod):
- 10 or higher → success. 9 or lower → failure.
- 3 successes → stable (still unconscious at 0 HP, no longer rolling).
- 3 failures → dead.
- Natural 20 → regain 1 HP and wake up immediately.
- Natural 1 → counts as two failures.
- Taking damage while at 0 HP → one failure (a critical hit → two failures; note any melee hit from within 5 ft is an auto-crit against an unconscious creature, so it counts as two).
- Instant death (massive damage): if a hit drops a creature to 0 HP and the leftover damage (beyond what brought it to 0) is ≥ its HP maximum, it dies outright — no death saves.
- Any healing brings the creature back to consciousness with that many HP. Successes/failures reset when you regain HP or stabilize.
Stabilizing an ally (no magic): an action + a successful DC 10 Medicine check (the "Stabilize a Creature" use of Medicine) makes them stable at 0 HP. A Sister of Battle's prayer, an Apothecary's narthecium, or a stim-injector that heals any HP wakes them outright.
ADVANTAGE / DISADVANTAGE
Roll 2d20 and take the higher (advantage) or lower (disadvantage).
- They do NOT stack. Three sources of advantage still = roll 2d20 once.
- If you have both advantage and disadvantage from any source, they cancel — you roll a single, normal d20 (no matter how many of each).
- Advantage/disadvantage applies to d20 rolls — attack rolls, ability checks, and saving throws. Not to damage.
Quick triggers: attacking a Prone/Restrained/Stunned foe in melee → advantage. Being Poisoned or Frightened, or attacking into long range / a Prone target at range → disadvantage. The Help action and many features grant advantage.
QUICK REFERENCE — THE NUMBERS YOU'LL FORGET
| Thing | Value |
|---|---|
| Default DC when unsure | 15 (Medium) |
| Critical hit | Natural 20 on an attack — roll the attack's damage dice twice (add modifiers once) |
| Critical miss | Natural 1 on an attack — automatic miss (no extra penalty in RAW) |
| Passive score | 10 + all modifiers (no roll) |
| Standing from prone | Costs half your speed |
| Difficult terrain | Every 1 ft of movement costs 2 ft |
| Long rest | ~8 hrs → regain all HP, half your hit dice, removes 1 exhaustion |
| Short rest | ~1 hr → spend Hit Dice to heal (roll + CON mod each) |
| Two-weapon fighting | Light weapon in each hand: bonus-action off-hand attack (no ability mod to its damage unless a feature says so) |
| Unarmed strike | 1 + STR mod bludgeoning, or grapple/shove |
DM'S OPERATING NOTES (read once, then trust yourself)
- You are the players' biggest fan, not the rules police. "Yes, and…" / "Yes, but…" beats "no." Reward clever 40k plans with advantage or a lower DC.
- Don't know a rule? Make a fast, fair ruling out loud and move on — "Let's call it a DC 13 DEX save; we'll check the book at the break." Momentum beats a five-minute page-dig every time. A confident temporary call is the right call.
- Fear the swarm, not the boss. Six Grots get six turns of attacks; one Ork Nob gets one. Many weak enemies are deadlier than the XP math suggests — and a lone boss gets dogpiled. Adjust mob size first to tune difficulty, HP second, damage last.
- Narrate the result, not just the number. "17 hits — your chainsword screams through ceramite and the traitor staggers, leaking black ichor."
- Spotlight everyone. Rotate who gets the moment; call quiet players by name.
- Protect the ending. If you're running long, cut a fight — always reach the climax. "The Emperor protects." So do you.