Cthulhu is considered a demon for game purposes; however, he is not covetous, and so will never teleport to meet the player. He can follow players to other levels, however. Unlike most other &, he does not respect Elbereth. When killed, he does not actually die. Instead, he turns into a stinking cloud, which will reform into him in only a few turns.
Although normally only seen in the Lethe patch, Cthulhu is in fact fully implemented in SLASH'EM. It is therefore quite possibly to create him in wizard mode. One cannot see him in a normal game, but it is possible to wish for a statue of him. Casting stone to flesh on said statue will create a doppelganger imitating his form, with all his attacks. Killing this doppelganger before it shifts into another form (a very difficult feat, although perhaps possible with firearms) does not do anything special, however.
Cthulhu's combat power is unreal; he is more than capable of dealing 100+ damage to a character with a full ascension kit equipped through melee attacks alone. In addition, he casts spells, confuses with his gaze, can eat your brain in the same manner as mind flayers, and disintegrates into a stinking cloud when defeated, only to rise up again in mere turns. A player of the Lethe patch is well-advised to kill him once, preferably from range, quickly retrieve the Amulet, and run very, very quickly. He is vulnerable to all elemental attacks, as well as magic missiles, so spellcasters should be able to muster up the force to kill him. Fighter-types would be advised to train with a ranged weapon beforehand, or possibly to simply use firearms. The only saving grace of Cthulhu's menace is that, unlike other demon lords, he cannot teleport. This should give you time to take him out from afar.
Encyclopedia entry (SLASH'EM)Edit
"The Thing cannot be described -- there is no language for such abysms of shrieking and immemorial lunacy, such eldritch contradictions of all matter, force, and cosmic order. A mountain walked or stumbled. God!... the Thing of the idols, the green, sticky spawn of the stars, had awakened to claim his own. The stars were right again... great Cthulhu was loose again, and ravening for delight." [ The Call of Cthulhu, by H.P. Lovecraft ] The exact origins of Cthulhu are lost to time, but it is known that in aeons long past a race of space faring beings came to this world and Cthulhu was amongst their number as the high priest who interceded between them and the dark gods they worshipped. Whether Cthulhu is a long lived individual or a title of office, is not known, but a creature bearing this name lives on, trapped in eternal slumber in the ruins of his great city.