The Dimensional Sanctum is a small uberlevel accessible from Caldeum via a portal. It contains the Creature of Flame uberboss and astral guardians which protect it.
Horazon the Summoner summoned a powerful demonlord that was too strong and overwhelmed him. He managed to trap it in his arcane sanctuary, but could not defeat it and it almost killed him. To save his own life, Horazon split off the prison zone from the rest of his sanctuary and banished it to earth where it now sits in a subterranean dimension. Breaking in and defeating the demon may yield great rewards.