Asmodeus, &, is the most powerful of the demon princes who is guaranteed to appear in Gehennom. He will generally be encountered in his lair, unless you have somehow managed to meet him before reaching that level. He is no trouble at all, provided you have decent AC and cold resistance.
Alternatively, if you are not wielding Excalibur, you can pay Asmodeus instead of fighting him. He asks for a portion of your visible gold so it is best to hide most of it, for example in your bag of holding.
Asmodeus will appear before you first and (if you aren't wielding Excalibur) demand that you pay him for safe passage. When you refuse, or if he fails to see where you are, he becomes angry. Like some quest nemeses and other named demons, he will teleport to the upstairs when he takes a few hits. The easiest way to defeat him is to lure him down one level and then stand on the upstairs. A less effective method is to drive him upstairs until you reach a level that allows you to teleport to the upstairs ahead of him. Note also that level teleportation can be used to lure him to a level that permits teleportation.
Alternatively, you can use a cockatrice corpse to turn him into a statue the first time he shows his face, and blast it for his wands and scrolls.
It is imperative that you be able to keep Asmodeus in your line of sight as much as possible, for like the aforementioned quest nemeses and named demons, in a few short turns after teleporting away from you, he will restore himself to perfect health, which is to the tune of 200 hitpoints.
It is said that Asmodeus is the overlord over all of hell.
His appearance, unlike many other demons and devils, is
human apart from his horns and tail. He can freeze flesh
with a touch.