A malevolent creature that took the form of Cordan’s mother, albeit with black buttons for eyes, Other Mother tried to capture Cordan, Xell, and Roknal and trap them in her realm. Using the bladed dagger to create rips that linked her realm with a darkened mirror world, they managed to defeat her guardians and escape her.
While Cordan and Xell were in the watery demiplane fighting with the elven runethane, Other Mother got the bladed dagger from the Keeper. She used its power to escape her prison and managed to displace the Lady of Pain, taking control of Sigil. Over the next several decades, she amassed an army and tried to subjugate her enemies and conquer the gatetowns. She eliminated many of the factions and other opposition forces, and was growing alarmingly powerful.
Cordan then escaped from the watery realm (and found that a century of time had passed by him) and used his powers as the true guardian of the bladed dagger to follow the rips Other Mother had made back to a strange demiplane that was constructed of Other Mother’s memories. In it they defeated an incarnate that had corrupted Other Mother, destroying her as well.