Asmodeus
By default only usable in the Nether. An altar made of 5 Obsidian pillars, the 4 outside pillars are 4 blocks tall topped with Diamond Blocks and at the center, a 5 block tall pillar with an Aberrant Soulcube at its core, must be 5000 blocks from spawn. Upon activation with a Soulkey, the Asmodeus Boss Event will be activated. Will spawn a large structure with the boss at the center.