Story related, cannot be missed.
Once you enter the Kingdom of the Dead, you will need to traverse several areas including The Breach until you reach the Serpent's Peak. From here (and after a really cool cutscene) you will ascend the Eternal Throne to speak with the Lord of Bones. Once you reach the summit, the king's chancellor will greet you and inform you that to see the King, you must compete in the Arena. Once the cutscene and your conversation with the Chancellor is finished the achievement will unlock.
A quick note that once you reach the Summit you will be able to speak with the second trainer in the game and unlock the Full Potential achievement (once you have the finances for it).