This is a normal-looking collar, with what looks like metal lining it. It feels like a metal-like leather that looks like they can swing open, but there's no separation where the holes would be. The collar is very small, it stretches and fits snuggly to whoever wears it. Its hight is also thin, a normal person could probably wear 5 pretty comfortably and without issue.
If your role high enough, the DM can decide to tell you about how "this collar has an adamantine wire that opens and closes the collar" or that "it looks like it could contain a spell".
This collar can contain a spell that affects the wearer in some way, that activates when the collar is put on. A spell that is cast this way has no duration, no material cost, and is cast instantaneously infinitely.
This collar is impossible to remove with pure force, dispel magic can only dispel the spell that was cast on it, the only people who can take it off is someone with a "key" that can take it off.
The final feature is that anyone can put it on or put it on other people, by throwing it at someone or yourself.
Notes: Control, Buff, Debuff, Utility, Necklace, Headwear, Outerwear, Artificer, enchantment
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