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.

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

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:

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:

• The first stage rolls the odds of Difficulty / (Difficulty + 10) to determine if an enemy should spawn on the current floor.
• The second stage will run a check if currently spawned enemies on a floor is lower than 3 + (Difficulty / 1.5), and will guarantee to spawn enemies if this number is not met.
• This is limited by the amount of tiles that are on the level, meaning that the second spawning stage will be capped out eventually and will be equal to the number of valid tiles.
• The third stage takes place only if running Crown of Blood. It will roll the odds of Difficulty / (Difficulty + 8) to spawn additional enemies on the current floor.

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.