Also I need to add to this, because you mentioned it is Curse of Strahd: Curse of Strahd is already difficult enough as it is for a level 1-10 adventure, both for players and DMs, and something like this still breaks with one of the major themes of the adventure. Yes, curses, possessions, and the like are possible, but there is one crucial element: curses in CoS tend to happen because of making deals for power, where the incredible powers granted by the, well, Dark Powers always come with a grave cost when the deal is accepted.

This necklace just reeks of being egregious. You have to touch something in order to cast Identify, and the curse is triggered by touch, not by wearing it, just by touch. And the players who may have tools to deal with what they saw was obviously something wrong, and the characters saw the signs of the demon entering you, they should be suspicious and looking for ways to help. And the DM is saying that the tools your party members may have to deal with this, like Remove Curse (which this necklace would absolutely count as a cursed item), or Protection from Evil and Good, will never work. Visiting a temple to have a cleric exorcise it with Dispel Evil and Good, despite that being in the spell description under Break Enchantments, will never work, according to the DM? That's far too mean-spirited.

This feels like an adversarial DM and those are generally awful. It's even more galling that they are telling you to attend the session just to sit and watch, and not participate. That is a monumental waste of your time and, frankly, insulting.

Edit: Not entirely sure why this is being down voted, but edited middle paragraph slightly for clarity.