Titans

The Titans are Ancient Ones, Nature Spirits who have gone bad, who have abused their power and and have been corrupted beyond repair. The only way to cure them, is to summon and defeat them.

Being in close relation with the elemental dragons, they are powerful beings, who will stop at nothing to destroy you. Once you summon them, there is no going back. You have to fight them, or you have to deal with them forever. If they manage to kill you, you will still have to deal with them. Their intention is to make you rage quit and corrupt your worlds. Hint: Don't let them win. If you summon them and sign off the world, your world will be overrun with hostile spirits and broken, corrupted code, making you have to delete that world, and maybe Minecraft itself.