This is a pretty common trope that’s related to the behelit myth, but I couldn’t find it on the tvtropes site. Here’s how it goes.
1. Someone, often an authority figure, comes into possession of a cursed something-or-other. Most often it’s a stone or amulet of some kind, but it could be a cursed book, cursed mask, etc. The most precise word would be the old sense of the word “fetish” to describe a pocket-sized idol (https://www.etymonline.com/word/fetish), but even my autism has limits.
2. Carrying it, they slowly become corrupted and other people become afraid of their changed behavior.
2.5 (Optional) The barony or local area becomes dark and toxic and/or full of tyranny and monsters.
3. The corruption eventually manifests as full-on demonic possession.
4. A crisis drives the possessed person to a proper Faustian bargain. Sometimes they’ll eat the bauble or, in a science fiction setting, inject it.
5. Their body transforms into a demon (often precedes a boss fight)
5.5 (Optional) After the hero wins, the barony or local becomes free and bright again.
Examples and near-examples:
Behelits in Berserk
Holy stones in Final Fantasy Tactics
The demons in Breath of Fire 2 (this game is full of interesting mythical shit)
Shards of the shikon jewel in Inuyasha
Plagas in Resident Evil 4
Imitation philosopher’s stones in Fullmetal Alchemist (first series, less so in Brotherhood)
The three Spiritual Stones and six Sage Medallions in Zelda Ocarina of Time
Archetypal example from Final Fantasy Tactics:
Archetypal example from Berserk, from this page through to page 150:
If you think that’s a downer and you want more, read to the end of the chapter!
Often the bauble is part of a larger bauble as in:
In this case it’s that whole prisca sapientia/life principle/will-to-power thing again.