Board Thread:General Discussion/@comment-10516645-20130614052342/@comment-6707115-20130617124951

Gallus was free to go and with an invitation to the Evergloom after Mercer had killed him only twenty years prior. Karliah even mentioned that the deal with Nocturnal in the afterlife only lasts until she feels compensated. So even if Nocturnal or Karliah didn't intend for a Nightingale to be free after their afterlife service was up, there's a loophole in the verbal contract.