The Sea Hag is a creature who lives in a lake. To summon her, a person must ring a special bell. The Sea Hag once claims that she too was beautiful once, just like her bell. Fifty years ago before the third book, she was offered the corpse of the Teleporter, Trope Kessel, by Batu.
Skulduggery and Valkyrie are after information about the murder of teleporters, and needing to talk to a corpse whos murder relates. They talk to Kessel and promise to take him back to land in exchange. The Sea Hag guards the lake, and refuses to let them. The Sea Hag trys to drown Valkyrie, but Skulduggery saves her and they plan to bury the corpse in a proper grave.
She appears taking Valkyrie from the Savagery. Valkyrie is saved by Una whom she later chokes for attempting to kill the Sea Hag. Una and the other Maidens of the Sea retreat. The Hag thanks Valkyrie for saving her life and gives her a bell she can use to call for her assistance.