EoC: Every dropped item is a demon eye.
King Slime: All liquid is replaced with thousands of slimes, Slime rain never ends and spawn rates are oncreased by 10000000%.
EoW: The whole world turns corrupt.
BoC: The whole world turns crimson.
Queen Bee: Bees will build hives anywhere they can, eventually covers the whole world.
Skeletron: Dungeon guardians replace enemy spawns.
Destroyer: Any minions turn into hostile probes.
Twins: All demon eyes/wandering eyes are replaced by EoCs.
Skeletron Prime: Mechanical guardians replace enemy spawns. Mechanical guardians spam the screen with lasers that do 999999 damage.
Plantera: Destroying plants summons an enraged Plantera for each plant destroyed.
Golem: All blocks in the world become unbreakable.Duke Fishron: Floods the world with water. (Like
@Yello_Xplorer said.)
Lunatic Cultist: All enemies gain the Lunatic Cultist’s AI.
Pillars: All enemy spawns are replaced by that pillar’s enemies and the enemy count to break the shield is increased to 10,000.
Moon Lord: All The Blocks in the world are destroyed.