That Which Is Not (a title they use in place of their name) is essentially a Stygian-verse Xsaiav'lairnn; a sentient eldritch entity that both represents, and is a part of, The Force of The Unknown. Not trying to be 'a person' in any way, they are entirely devoted to spreading, heeding, and becoming The Unknown, while doing what the Force cannot in the role of a 'sentient being'.
(note: there is never an official difference, but I've always thought of this character when solely named "Xsaiav'lairnn" to be more of an avatar of The Unknown, whereas with the added name "Mystrif" to involve a mage that was not originally part of The Unknown becoming a part of it. Usually the name "Xsaiav'lairnn" is taken by the mage after encountering The Unknown.)
This character is D&D verse Mystrif; a mage body that gets reassembled and strangely "possessed" by an eldritch force, until they both merge. The result is a being struggling to understand what it is to be 'a person', while trying to piece together now incomprehensible memories. Ultimately just goes full eldritch abomination in the end.