The Forgotten

The Forgotten is an entity from Mania that hunted Chaos Legion after they escaped its clutches with the help of Valadreth Rotweaver. They failed to remember it, as part of its nature was to only be recalled while being perceived, but Triquiis Fenwyn had numerous notes documenting its existence, though she couldn't fully comprehend it. She did eventually capture the creature, but it escaped its cage while in Prime and has not been uncovered since.