terraria how to awaken slime king?

Answer 1

king slime has a chance to spawn instead of a normal slime while you are at either end of the map on grass, where normal slimes can spawn, chance is about 1 in 300

or you can summon it with a slime crown (gold crown and 99 gel, made at demon alter)

gold crown made from 1 ruby and 30 gold bars at an anvil