"Avalon Guards have reported that an entity calling itself the Skeleton King has awakened in the lower levels of the Old Crypt. I knew this would happen. My brother Xanthar has been experimenting with Genetic Engineering... he found out about the ancient technology buried beneath Avalon, and has been trying to breed his own army. He doesn't understand the power he's tinkering with, and he himself has... changed. It's as though he's being controlled by something. Because of this, I had no choice but to imprison him in the Avalon Jail... indefinitely. Actually, it's all unfolding just like The Prophecy foretold. If you want to learn more about The Prophecy, check out the chest behind my throne. When you're ready, gather a team of Guardians, go to the Crypt, and defeat the Skeleton King."
This quest marks the first boss fight you will participate in within the quests. Before heading to the Crypt, you can look behind King Xero's throne and find a chest containing three books, each a different part of The Prophecy. When you are ready to defeat the Skeleton King, gather at least three other Guardians and head towards the Crypt. Once inside, head to the back of the room and go down the spiral staircase. Keep walking forwards and down until you find the boss room. To begin the fight, walk up to the casket, and the Skeleton King will spawn. When you and the other Guardians have successfully killed him, wait until you have been teleported to the loot room and claimed your loot before returning to King Xero at /warp throne.