CONCEIT: Withers was set to go down to Faerun. Ao said no, and that he should have had a Chosen. As it so happens, there was one in his Archives who had beef with Karsus. After eighteen hundred years of doing Jergal's (then Myrkul's, then Cyric's, then Kelemvor's) paperwork, he is set loose with minimal instruction. Chaos ensues.
OR: Jergal has a useful almighty idiot. He opens the door to let him out. Many Squids Were Harmed In the Writing of This Crack.