The Tower of Incarnate Pain (no relation to Her Serenity, the Lady of Pain) is a structure being secretly created by the yugoloths of Carceri. The current master of the tower is Bubonix. Rumors claim that when it's done, the entire plane will be under 'loth control and that, moreover, it will interface with the other two yugoloth towers (Khin-Oin and the Tower+Arcane) to make the 'loths the unstoppable masters of all the Lower Planes.
The gehreleths war with the yugoloths to prevent the Tower's completion.
Faces of Evil: The Fiends
Dragon Annual #2