The Placid Gene makes it so the mob that has it is (usually) incapable of attacking anything.
Specifically, the way it works is that anything that uses the TargetGoal#canContinueToUse method will check for this Gene and return false.
If it doesn't use that method (for example, the Creeper's SwellGoal), then it will be unaffected.
Obtaining
By default, level 1 can be obtained from these sources:
- Cow: 3/20