Need Help Deciding on a Name for a Homebrew Monster

Is the idea that this creature will carry on forever in this form? Or that it will only do so until it accomplishes some specific "unfinished business"?

If it's the former, leaning into that aspect of it might make a good name. Eternal Paladin. Or Deathless Knight, specifically casting it in contrast to a Death Knight. Or Forever Knight, if you're not worried about getting sued by whoever owns the 90s vampire cop drama series with that name.

If it's the latter, the name could be focused on the fact that the creature has a specific mission to accomplish. Oathbound Knight, perhaps.

When wqgnarokkr said “unfinished business i instantly thought of Ghost Knight! Other names that came to mine were Holy Knight or Angel Knight.

You could try a name that doesn't have "knight" in it, like Eternal Guardian or something. Something more fun and evocative than Adjective Knight.