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 a container.
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 or egg 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; he starts with 198 hit points.
Magical cold attack
Asmodeus is the only monster in the game who uses a special, magical cold attack. When used in melee, this attack does 30d6 damage (31d6 if Asmodeus has increased his level to 50) , for a maximum of 186 points of damage. This damage is reduced by half spell damage, and completely nullified by cold resistance. Magic resistance and reflection provide no protection against this melee attack. The cold melee attack cannot freeze potions in your inventory. In a normal game, most players will have acquired cold resistance by the time they meet Asmodeus; however, if you do not (for example, because of conducts), then Asmodeus becomes a fearsome opponent.
Asmodeus can also use a ranged version of his cold attack ("Asmodeus zaps you with a cone of cold"). The ranged version only does 6d6 damage. The ranged version's damage is also reduced by half spell damage, and is nullified by cold resistance and by reflection. Unlike the melee version, the ranged version can destroy potions in your inventory; reflection prevents this destruction.
Since Asmodeus wants the Amulet of Yendor as much as you do, you can exploit this behaviour. If you can lure him to the Moloch's Sanctum, he will teleport to the high priest, engage (and hopefully defeat) him/her, take the Amulet, and go back to attacking you, allowing you to obtain the amulet without leaving the vicinity of the staircase.
In principle, you could wound him just enough to chase him up the stairs, and repeat on each level. This lets you avoid the mysterious force, but is generally not worth the effort; Asmodeus can be struck by the force too.
Asmodeus comes from Dungeons and Dragons, that in turn derived him from Christian demonology. In both sources, he is a very powerful demon.
The D&D Asmodeus is a humanoid of over 4meters/13feet, with very expensive clothing, is very hansom, red eyes, dark skinned, and with two small horns.
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.