Champions of the Titan's Eye
Sanctuary of Alithor the Herald
When held by Alithor, The Map reveals this point. With the aid of The Man in the Lonely Tower and some visual cues, Alithor remembers the sanctuary’s layout and the secret code he used (among other defenses) to seal and possibly defend the sanctuary.
However, it appears the defenses did not work perfectly. The lower levels are corrupted by strange undead and outsiders suffering from the Demon Plague, a vicious disease thought wiped out ages ago.