The Stone Circle is located in the northwest part of the wilderness map. It can only be entered by PCs who have satisfied the demented ratling (section 2.5.8). Furthermore, it can only be entered during the very last hour of Darknight, between 23:00 and 00:00. The best tactic is to wear seven league boots and walk in wilderness squares next to the stone circle. Keep a very close eye on the clock and note how much time each move takes so the PC can calculate his steps exactly. The message "One of the monoliths seems to contain a shimmering portal." is generated when the PC stands on the Stone Circle square when it is eligible for entrance.
The Stone Circle is located on a plain. There is an entrance tunnel guarded by two chaos knights who should not be a problem for a PC at this level. The doorway into the circle is guarded by two annihilators, who can annoyingly destroy the PC's equipment. Ideally the PC should be invisible and destroy them from a distance. The alternative should be to hit them as hard as possible with the best melee attack available.
There is a unique pool inside the Stone Circle. Drinking from it results in the message "You feel altered in an eerie way." It always causes ~2 corruptions.
The circle itself contains Keriax, the multi-headed chaos dragon in the center with eight writhing masses of primal chaos scattered about the walls of the room. It is important to dispatch Keriax quickly, as he has a variety of very nasty attacks. He is able to breathe bolts including corrupting bolts, confuses the PC and drains stats if engaged in melee. His melee attacks are not corrupting, although he has five attacks per round and uses poison. He has one of the strongest confusion attacks in the game, regularly confusing high Wi PCs with confusion resistance. He is vulnerable to confusion himself, however. He is vulnerable to the Acid Ball spell, stunning, paralyzation and dragon slaying ammo. Potions of blindness, invisibility and cursed potions of invisibility work on him. Killing him from a distance is highly recommended. His corpse grants large increases to Strength, Toughness, Mana and Perception, but also causes massive corruption - about 8 corruptions. If he is killed late in the game and a PC needs corruptions for an Avatar of Balance or Ultimate Chaos God ending, this is one way to get them quickly.
The objective here is the Crown of Chaos, which Keriax holds. As with all parts of the Chaos Trinity, it corrupts the PC merely by carrying it. Put it someplace safe until it is needed.