h / & Cthulhu (pronounced by most people as "Ka-thu-lu") is not a monster found in vanilla NetHack. In various variants, he normally appears in Moloch's Sanctum.
Cthulhu's combat power is unreal; he is more than capable of dealing 100+ damage to a character with 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.
Cthulhu is considered a demon for game purposes; however he is not covetous, and so will never teleport to meet the player. Also, unlike most monsters of similar power, he does not fly or levitate. 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 possible 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.
A player of the Lethe patch is well-advised to kill him once, preferably from range, quickly retrieve the Amulet, and run very quickly or preferably use a cursed potion of gain level. 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 or fly. This should give you time to escape if necessary, and with careful planning may allow you to trap him where he cannot attack you in melee, allowing you to mow him down with ranged attacks with impunity. Moloch's Sanctum itself contains no water or lava, but if you lure him up one level you can trap him on the vibrating square easily, though you will be hit in melee a few times.
While he is not tameable in his normal form, polymorphing into a genetic engineer will allow you to turn him into something that can be tamed. This will eventually revert to being Cthulhu; as a pet, he is actually quite useful, doing unreal amounts of damage, not being covetous, and being fast. Also, taming him provides the only way to get rid of him permanently: he will not be able to revive if displaced into lava.
Here, Cthulhu replaces the high priest of Moloch as the guardian of the Amulet of Yendor. He is potentially even more dangerous than in SLASH'EM as he is covetous and able to teleport to the player's location; however, as he is not considered a demon in UnNetHack, wands of death or finger of death will work to kill Cthulhu. He also has an attack that makes the player lose memory of "25% of levels and 25% of objects" according to the sourcecode.
In Slash'EM Extended and SLASHTHEM he is a sidekick to the high priest of Moloch. He is the guardian of the amulet of Yendor, while the high priest will have a fake one.
Cthulhu comes from the works of H.P. Lovecraft. The encyclopedia entries are excerpts from Lovecraft works. Cthulhu is an important monster, giving its name to the "Cthulhu mythos" universe.
According to Lovecraft, "Cthulhu" was meant to be pronounced "Khlûl′-hloo". At the same time, however, he claimed that there is no "correct" pronunciation, since the name comes from an alien origin and includes sounds that humans aren't capable of producing. Most people just pronounce it "Ka-thu-lu".
He appears gigantic, hundreds of meters tall, humanoid, an octopus-like head with an undefined number of tentacles, a pair of rudimentary membranous wings and a scale covered body. His ability to disintegrate into a cloud when defeated, and then reform himself, comes from Lovecraft's story "The Call of Cthulhu".
"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 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