Legendary perks are available for high-level characters. See how Legendary perks can benefit your build and how many Perk Points you need to acquire them.
Mutations are semi-permanent modifiers that provide both positive and negative effects. They also affect Weapons and Armor with "Mutant's" legendary effect. A mutation can be acquired by consuming a serum or randomly through radiation exposure.
Addictions don't provide any positive effect, but they affect Weapons and Armor with "Junkie's" legendary effect.