Mother Malice

The grotesque sister in the family portrait Helena saw while banished into Dirtha's shack — described by Helena, with characteristic generosity, as "the grossest thing ever, the baby of a toad and a bag of garbage."

A hag, presumably, of the same dread line as Auntie Dirtha Bonebender (Imetka) and Granny IceTooth.

At the Pyramid: When Helena lifted Dirtha's bone deck from the sand after the hag's defeat, the chamber chilled and Mother Malice stepped through the wall alongside Granny IceTooth. She walked across the chamber to the heap of sand where her sister's skull lay, opened a mouth that was nothing of any honest creature, and swallowed Dirtha whole. Then she straightened, and both sisters vanished.

Whether she has taken her sister's body for grief, for nourishment, for retribution, or to keep something of Dirtha's reach in the world is unknown.