When a DM creates anything for their campaign, they should use the full weight of their imagination, in my view, not follow rules that are designed for player characters. Neither NPCs nor items created for a DM’s campaign should be thought of as limited by those rules, to me. It is actually a new concept to me that people regard this topic in a different light; quite an eye-opener. To the extent that many responses here and in other threads recommend attacking the DM, verbally or physically! I assume those are not literal, but just expressing strong feelings on the issue, but it is still a big surprise to me that players would be upset at a DM for keeping things fresh and different. I understand that in this case, it is also dangerous, but I am suggesting that the broader fact that it is a new and different challenge at least be taken into account.
There are some good suggestions on handling the power of this opponent above. Or, possibly, your group can’t defeat them. Having people in the world more powerful than yourself is pretty normal. Is this worse because they have a new magic item than if they were an exact PC-type character with several of the more deadly items in the DMG? I don’t see the difference at all, myself. Unless it is this determination to bind the DM with the rules that exist only to standardize player characters. They are not there to standardize other people in the world, in my view, but I do see that that is more and more the tendency in products. Really an unfortunate approach to playing an imaginative game, I think. Having a right to expect physics in the game world to remain consistent is I think pretty reasonable (with room for exceptions), but expecting that all magic will adhere to the options available to PCS seems like it would terribly limit the degree of imagination in the game, to me. Sometimes liberal use of their imagination by the DM will benefit the players, sometimes their opponents- but the game would be awfully repetitive and dull if the DM did not have freedom to apply their ideas.
If the DM’s rule explanation for this magic item seems weak, maybe remember that they don’t really have to have one at all. The fact that a PC can’t duplicate something does not mean that it can’t exist. Focus on in-game thinking instead- it is a new powerful magic item, not a real surprise in a world that has plenty of such things.