We all know that at the end of the Octo Expansion, Tartar possessed Three for a while, was eliminated by Pearl, Eight lived a happy life on the surface, etc. But what if Tartar survived? Not in a general "oh he somehow survived the Killer Wail", what if he was able to rebirth himself via growing inside Three? That's what this fic is about. NOTES: - Three uses She/Her - Tartar's clone body uses She/Her and presents as a woman - Tartar's old body uses He/Him and is very much not alive - Tartar is a parasite, like Venom in a sense - Bold text is Tartar, standard text is Three