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To clarify what I mean here I said that Darth Nihilus in my weird head-canon/fanfic was originally Mandalore the Ultimate BEFORE he became Darth Nihilus, the leader of the Mandos that started the Mandalorian Wars, and I am trying to apply logic to that. As for your idea though, I always wondered if Nihilus can and have lived on in his mask and possess people like Momin, (the Sith you mentioned), from the New Canon, or if there is something else going on. There is a Tabletop book called "Unknown Regions" and one of its mini-adventures is called the "Mask of Darth Nihilus". In that mini-adventure, it has interesting lore bits on Nihilus's mask, but it doesn't seem like he outright possesses people, at least from what I have gotten from it, but who knows. Regardless though, this is a headcanon and theory thread so talking about what seems "official" or not is irrelvant to the topic of the post, (especially since Mandalore the Ultimate is for sure not Nihilus in the real KOTOR lore but he is in one of my fanfic/headcanon idea story).

I don't know if you have played the Fable games, but Jack of Blades in that franchise is kind of a demonic entity that inhabits a mask and can take over the host when worn. I have always liked the idea that Darth Nihilus became like that, his hunger so great it eroded his physical form and that his spirit attached to the mask. In the new canon, there is a Sith who is also attached to a mask, so this is a way we could get Nihilus back.