Difficulty Explained


Difficulty is a hidden number that is used by the game to determine when weapons are added to the drop pool, and how many enemies will spawn during level generation before cluster spawns are accounted for. The higher the difficulty the more enemies that can spawn, but also the possibility of higher-tier weapons appearing.

Difficulty starts at 1 on 1-1 and is incremented by 1 every time the player enters a portal. 1-2 would have 2 difficulty, and 1-3 would have 3 difficulty, and so on.
With no secret areas this would give 15 difficulty at the end of preloop, and then +16 difficulty for every consecutive loop completed.

As difficulty is increased based on portals entered that means that secret areas can situationally increase/decrease difficulty relative to a normal run.
If any levels are skipped (primarily with Oasis) this will result in less difficulty meaning that areas will have more sparse enemies, but also delay how soon better weapons can drop.
Any additional levels (Pizza Sewers and Crown Vaults) will cause for additional difficulty which can add more enemies to a level, while also adding better weapons to the drop pool sooner.
Replacement areas (Y.V.'s Mansion, Cursed Crystal Caves and Jungle) will keep difficulty the same relative to a normal run.

A normal run has 15 difficulty by the end of preloop, so if a run visits Pizza Sewers (+1) and both Crown Vaults (+2) it would have 18 difficulty instead.
A full Oasis skip would miss 1-2 to 3-2 which skips 5 areas, while Oasis gives +1 back, so it would be -4 difficulty resulting in 11 difficulty by the end of preloop.

There are two niche cases that effect difficulty, the first being IDPD HQ which can be entered and then exited for extra difficulty. This dropdown will explain it in full but is not necessary to read for the majority of players.

IDPD HQ

As a note HQ is only mostly understood, so there may be some discrepancies as behavior will change based on version.

If IDPD HQ is entered it will take the player to HQ1. A Van can be taken to then exit again, or the level can be completed to visit HQ2. If the Van is not taken in HQ2 to leave, then the run will end on HQ3 with either victory or death. HQ can only be visited once a run and only during Loops 1 or 2. Visiting HQ for extra kills/difficulty is known in the community as "HQ Abuse".

For Non-Beta (u99r1) HQ will behave differently based on levels entered.
Entering HQ from a transitional area (2-1, 4-1 and 6-1) will simply bring the player to the next area, such as 2-1 -> HQ -> 3-1. The same applies to any X-2 areas (1-2, 3-2, 5-2 and 7-2), and should apply to any secret areas with Vans.
Under this condition HQ1 would give +1 difficulty, while HQ2 would give +2 difficulty.

If HQ is entered from the first level in a larger area (1-1, 3-1, 5-1 or 7-1) it will then skip a normal area, bringing the player to the X-3 area instead. An example would be 3-1 -> HQ -> 3-3.
For HQ1 this will maintain difficulty similar to replacement areas, while HQ2 will give +1 difficulty.

HQ Abuse is mainly done on X-3 for Non-Beta as it will repeat that level. For example going from 1-3 L1 -> HQ, exiting HQ causes 1-3 L1 to repeat. Level generation may change for the repeat level.
For HQ1 this will give +2 difficulty, while HQ2 would result in +3 difficulty.

For the Betas (2021/2025) and NTT v100 HQ will no longer repeat or skip levels. Exiting HQ will simply bring the player to the next area, such as 1-3 -> HQ -> 2-1.
This will always result in +1 difficulty for HQ1, or +2 difficulty for HQ2.

The only exception on the Betas is if a Vault portal is open on a X-2 level, then entering/exiting HQ will cause the X-2 level to be repeated. This is difficult to do as Vault portals will prevent Vans from spawning in, so the statue must only be broken once the Van's spawn initiates.
For HQ1 this would give +2 difficulty, while HQ2 would give +3 difficulty.
(It is unknown if this works on Non-Beta as well, needs testing.)

Score chasers on Non-Beta will either do HQ abuse from 1-3 L1 which is the earliest opportunity for maximum extra difficulty, or 1-3 L2 for better chances of increased kills with some increased risk. On the Betas this is usually just done on 1-1 L1 to increase difficulty as soon as possible, but could be delayed until 1-2 if wanting to attempt the extra difficulty as well.


The other niche case is Y.V.'s Crib which will be expanded upon another day, as it will take time to fully document.

Y.V.'s Crib

Coming soon to a Wasteland near you!

The other factor for difficulty is if playing Hardmode. This will increase difficulty by 2 instead of 1 for enemies, yet weapons will be added to the drop pool at a much slower rate. It is fully explained on the hardmode guide (work in progress).


Weapon Drops


Here is a list of weapons for illustration, from the weapon table page.

Weapon drop logic relies on difficulty to determine what weapons are allowed to drop at any point. As difficulty naturally increases more weapons are available to drop, creating a steady power progression in the run.

Internally every weapon has a set tier, a number used for comparison against difficulty. If a weapon's tier is higher than the current difficulty it cannot drop, but if it is equal or lower to the current difficulty then it can.

All weapons have an equal chance of dropping as long as they are in the drop pool. Weapons that are currently being held are removed from the drop pool which gives slightly better odds to see other weapons. (Weapons are not removed from the drop pool for Steroids due to his dual wielding.)

There are multiple sources in which weapons can drop and some of them effect difficulty differently. These sources include:

IDNTG listing: In actuality:
  • Enemy Drops: -1
  • Weapon Chests: +0 (Default)
  • Big Chests: +0
  • Cursed Chests: +2
  • Big Cursed Chests: +3
  • Crib Giant Chests: +5
  • Gun Gun: +9
  • Enemy Drops: +0 (Default)
  • Weapon Chests: +1
  • Big Chests: +1
  • Cursed Chests: +3
  • Big Cursed Chests: +4
  • Crib Giant Chests: +6
  • Gun Gun: +10
IDNTG lists weapon chests as the default which is different then what it is in actuality. This is done for simplicity's sake as weapon chests are guaranteed to spawn every level and can be treated as the default, while not technically correct.

Other things to note is that both cursed chest types will never include weapons that appear in 1-1 chests, which includes 8 weapons. Robot's passive gives them a +1 bonus to see weapon drops sooner, while their Refined Taste ultra will remove any weapon up to the normal 2-1 drop pool (the first 28 weapons).


Enemy Quantity


The difficulty number is also used by the level generator as the odds of an enemy spawning on each floor tile, and also determining the minimum number of enemies that must spawn.

Level generation will start by building the layout of the level and then populate it. Every floor tile will run the odds on whether it should spawn enemies or not. Floor tiles around the player's spawn will be skipped, and tiles containing weapon chests/rad cans will be skipped.

There are three stages of enemy population:

Cluster Spawns are not based on difficulty but rather simply loops, not occurring on preloop. After the level has been fully populated it will run the calculation on every enemy with the odds of Loop / 60. If successful it will add the same enemy types based on the calculation of Loop + 3.

Enemies that spawn midway into a level cannot include cluster spawns. Mimics, Health Mimics, and Maggot Nests are also excluded from spawning in clusters but could have in previous versions, while Hostile Horror can still spawn in a cluster.


Cluster Spawning Chart

Loop: 1 2 3 4 5 6 7 8 9 10
Odds: 1.67% 3.33% 5% 6.67% 8.33% 10% 11.67% 13.33% 15% 16.67%
Extra Enemies: 4 5 6 7 8 9 10 11 12 13
Loop: 11 12 13 14 15 16 17 18 19 20
Odds: 18.33% 20% 21.67% 23.33% 25% 26.67% 28.33% 30% 31.67% 33.33%
Extra Enemies: 14 15 16 17 18 19 20 21 22 23
Loop: 21 22 23 24 25 26 27 28 29 30
Odds: 35% 36.67% 38.33% 40% 41.67% 43.33% 45% 46.67% 48.33% 50%
Extra Enemies: 24 25 26 27 28 29 30 31 32 33