OnlineDM
Adventurer
I'm inclined to go with the folks who say to run it as-is. A level 22 versus a party of level 13 PCs should be able to destroy them handily, but not in one round. Make it clear in-game how scary this guy is. He's way out of their league. It would take a miracle for them to beat him in a fair fight.
Then, it's up to the party to decide what to do. They can decide that discretion is the better part of valor and choose to go elsewhere for now. They can try to make it an unfair fight - perhaps by asking for divine help or similar things, as others have suggested. They can attempt the fight, realize that it's hopeless, and retreat. Or they can fight and stand and die. It's up to them.
It sounds like you have a somewhat sandboxy game, which means that PCs might run into threats that they really can't handle yet by themselves. I think it's appropriate to embrace that.
Good luck, and let us know what you ultimately decide to do and how it goes!
Then, it's up to the party to decide what to do. They can decide that discretion is the better part of valor and choose to go elsewhere for now. They can try to make it an unfair fight - perhaps by asking for divine help or similar things, as others have suggested. They can attempt the fight, realize that it's hopeless, and retreat. Or they can fight and stand and die. It's up to them.
It sounds like you have a somewhat sandboxy game, which means that PCs might run into threats that they really can't handle yet by themselves. I think it's appropriate to embrace that.
Good luck, and let us know what you ultimately decide to do and how it goes!