The First Flensing was a powerful ritual that, if completed, would allow the evil god Asmodeus to enter Waterdeep. Wizards and clerics at the House of Wonder in Waterdeep first attempted the ritual around 1180 DR in exchange for immortality. However, they reneged on the deal and were cursed into lich form and became the Circle of Skulls. In 1480 DR, several parties, including the Circle of Skulls, attempted to complete the ritual but the deva Jinnaoth Ir'Gadohn halted the attempt.
The exact effect of the ritual was unknown. However, it appeared that the ritual would allow the god Asmodeus to enter Waterdeep and create Hell on earth. Because other deities were unlikely to allow Asmodeus free entry to the Prime Material Plane, completion of the ritual would probably lead to a war between the gods.