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When a Spore Mob kills another mob, they will receive 1 Kill Point and 1 Evolution Point. All Mobs (excluding Organoids) have an internal counting system for both and can use them in different ways depending on the Mob. Organoids don't get Kill or Evolution Points because they are only temporary and have natural Regeneration. Upon a Mob Evolving, they will keep any Kill and Evolution Points they had.
Kill Points
Kill Points can be obtained by different Mobs in multiple different ways.
1 Kill Point is gained by Spore Mobs (excluding Organoids) whenever they kill a mob. This does not count when they infect a mob.
4 - 6 Kill Points to Infected spawned by Overcharged Scents.
0 - 3 Kill Points by Calamities performing a Spore Burst to Infected, Evolved, Hyper Evolved, Scampers, and Gastgebers within a 16 block range.
1 Kill Point from Infested Spawners to Infected, Evolved, Hyper Evolved, Scampers, Gastgebers, Calamities and Structure Blocks every 18 seconds in a 32 block radius.
Specters can receive Kill Points by consuming Food in Chests.
Upon a random chosen tick, if the check for a certain Effect is made successfully, there is a ~5% chance the Mob will receive that Effect. The Effect can only be granted if the Mob doesn't already have it. Granting a buff will consume a Kill Point, and multiple buffs can be given at once if all the criteria are met. Spore Mobs can get the following Effects under the following circumstances:
if the Mob is below 100% health they will receive Regeneration for 10 seconds. For Specters and Infested Constructs, it will be 20 seconds of Regeneration.
If the last damage the Mob took was from drowning they will receive Water Breathing for 5 seconds.
If the Mob has more than two Kill Points and has a target, there is a 2.5% chance the following two checks will be performed:
Kill Points are also given to Structure Blocks by Spore Mobs when they are called over. Infected, Evolved, Hyper Evolved, Scampers, and Gastgebers will use Kill Points in these ways. Some specific Mobs can use Kill Points in unique ways.
Having Kill Points prevents Infected from Starvation.
Depending on the Mob's Kill Points when being fed to a Womb will depend on the amount of additional biomass gained.
Depending on the Mob's Kill Points when consumed by a Proto Hivemind will depend on the amount of biomass she gains when removing their Kill Points.
A Hohlfresser with Kill Points will have their vomit projectiles have bone pieces in the texture.
Evolution Points
Evolution Points can be obtained by different Mobs in a few different ways, albeit less than Kill Points, as they have less functions.
1 Evolution Point is gained by Spore Mobs whenever they kill a mob. This does not count when they infect a mob.
1 Evolution Point from Infested Spawners to Infected, Evolved, Hyper Evolved, Scampers, Gastgebers, and Calamities every 18 seconds in a 32 block radius.
If a mob from the Infected tier has at least 1 Evolution Point, they will Evolve after 5 minutes. If an Evolved mob that has a Hyper Evolved form has gained 7 Evolution Points, they will evolve after 5 minutes.
Hyper Evolved will use Evolution Points to place Human Remains at their Nests. This is because they have no use for them anymore.
Trivia
Spore Mobs can gain Kill Points and Evolution Points from killing other Spore Mobs, though making them fight is only achievable via commands or mods.
Checking for the amount of Kill Points prior to buffing only happens twice; before all logic is fired, and before checking combat-only logic. Due to this, it is possible for a Spore Mob to use more Kill Points than it has, resulting in a negative amount of Kill Points. Due to how the logic is constructed in the code, this is only possible if the Mob has only 1 Kill Point and grants itself Regeneration and Water Breathing in the same go.