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 night of Darknight. Determining exactly when it is available is not an exact science. In practice, the PC stands on the circle as soon as it is Darknight and repeatedly presses > until eventually he enters.
The Stone Circle is located on a plain. There is an entrance tunnel guarded by two dwarven 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.
2.23.1 Keriax
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. He is vulnerable to Death Ray from the spell or wand and to dragon slaying ammo. Potions of blindness 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.