"BWHAHA! YOU SHALL NEVER EXPECT TO LIVE, HUMAN BEING!" - Key Master
Key Master is a post-Plantera boss which spawns in the underworld using the legendary essences. He is the hardest boss pre-Moon Lord.
Defeating this lets you lift Key Master's curse and turn him into a NPC.
As said above, you need to use the legendary essences dropped by enemies. Once you get them, go to the underworld and find the Strange Key Altar.
Key Master is a gigantic cartoon looking monster with five keys. Like the fight with Zeus in Super Villain Island, he will repair the keys if they are damaged, and if destroyed they would make him lose some of his abilities. His normal attacks are incredible references to Kingdom Hearts. He throws keys, and also swings them. Frozen Key allows him to summon Frost Warriors which inflict Frozen when doing spin attack, Jungle Key allows him to summon Lava Sharks which inflicts knockback no matter if you have a knockback (Cobalt Shield, etc.) accesory. Corrupt Key basically lets him throw a yoyo which shoots cursed flames everywhere, or summon Corrupt Wisps. Crimson Key lets him create Magnet Sphere-like ichor projectiles, while also summoning Ichor Stickers. If you see him charging purple and red energy, that means he's gonna summon Desert Spirits. Hallow Key allows him to... PULL YOU IF YOU ARE ABOVE HIM. Well, with a rainbow hand. Once you destroy all of the keys, he will say "Forget about it, fool. YOU CAN'T BEAT ME!", and he will pull you to the surface where an epic battle would take place. He summons Jungle, Snow, Corrupt, Crimson, AND Hallow while in this phase. Once beaten, he will become uncursed.
In Expert Mode, moveset doesn't change, but are more POWERFUL.
He will drop one of the five biome weapons on defeat, dropping 10 platinum on defeat, and becomes uncursed.
Key Mastery - Defeat the Key Master and lift his curse.
Bad Day Scenario - Defeat the Key Master in Expert Mode.
Did I See THAT? - Try and respawn the Key Master and end up getting boned.
Sora-Ranked - Defeat the Key Master with only Biome Weapons.
Egg Hatching - Defeat the Key Master and get his egg.