King Slime: Increases jump height and safe fall distance greatly
Eye of Cthulhu: 2 boosters
1) Provides creatures, treasures and traps detection
2) Gives Shine and Nightvision effects
Eater of Worlds: Increases melee speed by 5%, movement and mining speed by 25%
Brain of Cthulhu: Increases max amount of minions/sentries by 1, Heartreach effect
Queen Bee: Hostile bees do less damage and your regeneration is increased by 6, immunity to poisons
Skeletron: 2 boosters
1) Skeletons contact damage is reduced, all damages/critical strike chances are increased by 10%
2) Provides immunity to fire blocks, Obsidian Skin, Gills and Flipper effects
Dark Mage (Old One's Army tier 1 boss): Increases magic damage by 25%, max mana by 50 and mana regeneration greatly
Wall of Flesh: Increases max amount of minions/sentries by 1, defense and DR by 10/10%
Pigron: Provides Well Fed
Ice Golem: Provides immunity to Chilled, Frozen and Frostburn debuffs
Goblin Summoner: Makes your attacks inflict Shadowflame and makes you immune to it
Twins: Archery and Ammo Reservation effects, immunity to Cursed Flames and Ichor
The Destroyer: Increases mining speed by 33% and increases max life by 25%
Skeletron Prime: Increases armor penetration and melee speed by 15/15%, gives 200% thorns effect
Ogre (Old One's Army tier 2 boss): Increases defense and damage reduction by 5/5% and provides knockback immunity
Plantera: Damages and critical strike chances are boosted while you are moving, Philosopher's stone effect
Golem: Increases attack speed by 10% and increases melee knockback
Betsy (Old One's Army tier 3 boss): Your attacks inflict Daybroken, flight abilities are increased
Duke Fishron: Gives ability to dash, all stats up while on surface
Martian Saucer: Provides immunity to Electrified and Distorted debuffs
Lunatic Cultist: Reduces damage taken from Pillars enemies, mobs may drop lunar fragments
Moon Lord: You emit aura which weakens enemies around