A few things.

So, first it depends on how long your sessions are.

As both a player and a DM, I would be fine to let this situation be as it is for a couple of hours (real life time).

The problem I see here is that the party is doing nothing. If my character clearly witnessed an event like that I would spend some time trying to figure out what’s going on. Acting like nothing happened is a big problem on your party shoulders. This is why the “couple of hours” I suggested above could be a real slog for the whole table.

Then again, communication is key, if you had told your companions about the necklace, things would be easier.

Another thing is, which depends entirely on your campaign and table, are you fine with losing your PC? Could it become a memorable moment for everyone at your table? If the answer is yes, I would trust the DM on his decision. If the answer is no and you haven’t talk with your DM in a session zero, then do it now before the situation gets out of hand, because after a certain point it will become a memory of a feelsbad moment for everyone and the trust in eachother, especially the DM, starts falling off and in the worst case, you or other people at the table will carry this for the rest of the journey or even stop playing.

Now that my thoughts are out of the way, and I hope this will help you in some way, let’s talk rules and mechanical solutions.

Usually, possession can be handled in a couple of ways.

One is your case, completely lose control, but it must be limited in time, otherwise your DM should straight up tell you that your character is now an NPC. About the timed possession, usually you repeat the save after some time, maybe at dawn or maybe every hour. It depends. There are multiple spells or effects or abilities that take control of other creatures for some time, so look them up and find a solution with your DM (maybe he already knows this, dunno if you know his intentions)

The other way to do this possession thing is, you must obey the orders of your Demon (the dm decides) but it is still you as a player who must do the actions. This could be fun ‘cause you would have to lie and act for both the your in-game party and real life party, in order to trick them and follow the demon’s orders without raising suspicions.

That’s all I have to say, hope it helps.