The Legendary Beast Invasion has a chance of occurring any day after the Moon Lord is defeated, with a 0.68% chance to occur each day. In this event, several Legendary Beasts spawn, with lesser beasts around the world to protect them.You have to fight these beasts in a specific order. First, you must defeat Shard, Beast of Darkness to receive an Amulet Piece, as this is the standard drop for all beasts.
After this event, the Shaman NPC will move in and start sharing some advice with the player.
Credit for all of the ideas goes to Anarchy44
Credit for all of the sprites goes to Anthony Plays