The Core Rules
The Core Mechanism: The Window of Success
Gloom is using a d20-based “Roll Between” system where the Difficulty Level (DL) of the task you want to succeed with is the floor and your Skill Rank (SR) is the ceiling. It avoids subtraction (which slows down play) and adds a narrative "push your luck" feel, perfect for hacking and high-tech combat. As a player you have to “beat the threshold” with your rolls. You have to roll high enough to overcome the enemy's tech (The Floor), but low enough to stay within your own control limits (The Ceiling).
Player Character (PC) has attributes and skills. They both have a number showing the PC:s ability in respective skill or attribute. The number usually ranges from 1-20 and you check your success when you attempt something by rolling a d20 against that number. To succeed, your d20 roll must land within the Window of Success.
The Skill Check
The Formula: Difficulty Level (DL) ≤ SUCCESS ROLL ≤ Skill Rank (SR)
You must roll UNDER your Skill Rank (SR) but ABOVE the Difficulty Level (DL).
The Check: Roll 1d20.
The Floor (DL): The Difficulty Level. Represents external factors (Enemy armor, lock complexity, Gloom turbulence). You must roll ABOVE this number.
The Ceiling (SR): Your Skill Rank. Represents your internal capability. You must roll EQUAL TO or UNDER this number.
The Goal: Roll a number between your Skill Rank and the Difficulty Level.
Failure: You roll BELOW the DL or OVER your SR.
Success: You roll EQUAL TO or LOWER than your SR but HIGHER THAN the DL.
Undershoot (Failure): You roll EQUAL TO or LOWER than the DL. The task was too hard for your effort. The targets armor was too thick. The firewall was too complicated.
Overshoot (Failure): You roll HIGHER THAN your SR. You panicked, lost control, or lacked the skill. Your hand shook just as you fired your gun. You were too confident when hacking the firewall.
| Example |
|---|
| You have HACKING 14. |
| You try to hack a door that has DL5. |
| Your Window of Success is 6-14. |
| You roll a 12. Success. |
| You roll a 15. Failure. |
The Extreme Outcomes a.k.a The Criticals
There are some special outcomes that can happen when you roll the die, both good and bad.
- Exact number of Attribute/Skill Rank: PERFECT SUCCESS. You succeed spectacularly. Double damage, instant effect, or you gain a moment of clarity (restore 1 Stress).
- One number below Attribute/Skill Rank: SPECIAL SUCCESS. Trigger Effect. Standard success + a specific bonus (Weapon/Class ability).
- Exact number of DL: SUCCESS with COMPLICATION. You succeed but there is a complication. You hit the target but the weapon jams, you lock the door but your coat gets stuck, you hack the firewall but trigger an alarm.
- Natural 1: CATASTROPHE (The Panic Trigger). You fail, and something terrible happens. Weapon jams, lock breaks, or you trigger a Panic Check.
The Mechanics
Let's define the Success Zones. Suppose a Player has Athletics 15 and is trying to climb a challenging cliff wall with DL 6.
| Die Roll | Result | Effect |
|---|---|---|
| 16 - 20 | OVERSHOOT | You push too hard and have to pause or climb back down. |
| 15 | PERFECT | Perfect Success. You climb the wall in no time. |
| 14 | SPECIAL | Trigger Effect. You climb the wall with no effort. |
| 7 - 13 | SUCCESS | Standard Success. You climb the wall. |
| 6 | COMPLICATION | Success with Complication. You succeed but something goes wrong. You drop your weapon while climbing. |
| 5-2 | UNDERSHOOT | Blocked. The difficulty was too high. You can’t find a way up and have to climb back down. |
| 1 | CATASTROPHE | You fail horribly. The carabiner snaps and you fall. |
Example Modifiers (Difficulty)
| Difficulty | DL | The Challenge |
|---|---|---|
| Routine | 0 | Opening a civilian door, shooting a target in the open. (Usually no need for a roll). |
| Standard | 5 | Hacking a local bank, shooting a target in light cover. |
| Hard | 10 | Decrypting military comms, shooting a moving target in heavy cover, storm in the Gloom. |
| Extreme | 15 | Hacking an AI core, shooting a thermal exhaust port while flying at Mach 2, “nightmare” current in the Gloom. |
| Impossible | ≥ Att. | If DL is higher than your Attribute, you have No Window. You cannot succeed without Gear or Aid lowering the DL. |
Advantage and Disadvantage
When you gain a Skill Level of 21 you become a master of that skill, (e.g. a Pilot flying a ship), and you gain Advantage. You can also gain Advantage by some cybernetics, psionics or special items.
Advantage: Roll 2d20 and take the best result.
If you gain too much Stress and Panic, you gain Disadvantage. You can also gain Disadvantage by getting a serious wound or by using improvised tools.
Disadvantage: Roll 2d20 and take the worst result.
Contested Rolls
The best way to handle contested situations where two persons are active, like dogfights, hacking duels, or grappling, is to use Contested Rolls.
Highest Successful Roll Wins
When two characters compete, they both roll a d20.
- Check for Success: First, both parties must roll UNDER their own Stat. If you roll over, you fail automatically.
- Compare: If both succeed, the person with the HIGHER die result wins.
How this works: In GLOOM, a result close to your limit implies you pushed your skills to the max without failing. A 14 is a "stronger" effort than a 3.
Example Scenarios
Scenario A: The Hacker Duel (Success)
The Player (Netrunner) is trying to steal data while a Corp Netrunner tries to boot them out.
Player (Hacking 16): Rolls a 14. (Success! A very strong attempt).
Netrunner (Hacking 14): Rolls a 5. (Success! But a weak attempt).
Result: 14 beats 5. The Player bypasses the Netrunner's ICE and steals the data.
Scenario B: The Dogfight (Failure)
The Player is piloting a fighter jet and trying to shake a missile lock from an Enemy Ace.
Player (Piloting 15): Rolls a 18. (FAIL! They rolled over their stat).
Enemy Ace (Piloting 13): Rolls a 4. (Success).
Result: The Player failed the maneuver entirely. The Enemy wins by default. The missile hits.
Scenario C: The Messy Brawl (Both Fail)
Two drunk space marines try to punch each other.
Marine A: Rolls Over.
Marine B: Rolls Over.
Result: A stalemate. They both swing, miss, and stumble. The situation doesn't change.
Advanced Variation: "The Dynamic Floor"
If you want to make combat feel really reactive, you can use the Defender's Roll to set the Attacker's Difficulty (Floor).
This is great for "Active Defense" (Parrying or Dodging).
How it works
The Defender rolls first.
Whatever number they get (assuming they succeed) becomes the Difficulty Floor for the Attacker.
The Attacker must roll UNDER their Stat, but OVER the Defender's result.
Example: The Sniper Duel
Target (Defender): Tries to move erratically. Rolls Interface check (14). Result: 11.
Sniper (Attacker): Has Interface 18.
The Shot: The Sniper must now roll between 12 and 18 to hit.
If the Sniper rolls a 5: It’s a miss (The target moved too well).
If the Sniper rolls a 15: It’s a hit (Pure skill overcame the defense).
Handling Ties
What happens if the Player rolls a 12 and the Enemy rolls a 12?
Option A: The Stat-Breaker (Recommended)
The character with the higher Skill Rank wins.
Player (Hacking 16) vs Enemy (Hacking 14).
Both roll 12.
Player Wins because their raw talent (16) is higher.
Option B: The Defender Wins
In a tie, the status quo remains. The attacker fails to change the situation.
