Great Cthulhu is a monster from the dNetHack variant.
Great Cthulhu is the second or third most dangerous creature in dNethack, and as such should be avoided rather than confronted. Great Cthulhu is generated asleep on one of the altars in R'lyeh, and plan A should always be to keep him that way. Great Cthulhu can be sensed from standard warning distance (13x13) as "an unknown monster causing you dread" regardless of whether or not you have warning. Beware of squeaky boards near the upstair, these can cause him to awaken. You will also want a wand of opening to lower the drawbridge, as striking will make a loud noise.
Should the character need to approach Great Cthulhu, it is vitally important to avoid looking at him. Extinguish light sources and remain out of sight or blind yourself. Great Cthulhu's gaze attack permanently reduces wisdom by 1d10 points, though not below 3. In this case, the character's wisdom is reduced to 3 and the character then takes 10 damage for each point of wisdom drain remaining. You will also suffer amnesia, including losing 25% of all spell memories. For this reason, it's best not to light up the level (it is naturally dark). Sustain ability prevents both the drain and damage, however not the amnesia.
It is vitally important to remain at least 2 squares away from Great Cthulhu if at all possible, as his melee attack deals 400 damage and is essentially guaranteed to hit.
Should combat become unavoidable, Great Cthulhu is actually decently easy to take down, thanks to his slow effective speed (Speed 3, despite the listed speed of 15) and lack of a ranged attack. Should he be stricken down, however, he will revive at full strength after only a few rounds. He also disintegrates into a stinking cloud, and which may hurt non-poison-resistant players (though you should have poison resistance by this point). However, he will keep pursuing and warping to you.
Because of this, it's best to have several potions of full healing, an amulet of lifesaving (multiple is highly preferred), a source of sustain ability, and very fast speed. Do not rely on spells, as the amnesia causing passive attack will cause you to forget those quickly. If you use a potion of object detection to find the Silver Key, make sure you make a note of where it was. Amnesia will cause you to forget its revealed location. If Cthulhu is awakened, your best strategy is to grab the Silver Key and sprint away as fast as possible. You shouldn't need the 3rd Key of Neutrality, any character capable of making it to R'lyeh is more than capable of getting another alignment key.
Great Cthulhu is just an alternative name to Cthulhu.
Cthulhu comes from the works of H.P. Lovecraft. The encyclopedia entries are excerpts from Lovecraft works. It's an important monster, giving it's name to the "Cthulhu mythos" universe.
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 in a cloud when defeated, and then reforming him self, is in the story.
"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
I shall never sleep calmly again when I think of the
horrors that lurk ceaselessly behind life in time and in
space, and of those unhallowed blasphemies from elder
stars which dream beneath the sea....
[ The Call of Cthulhu, by H.P. Lovecraft ]