In our 5e game, I was put on the spot for some loot the players were promised by a Hill Gnome. When it came time for their reward, I made up a dagger that can paralyze whatever it stabs, gave it to our Dragonborn Fighter, and it really threw off the balance of the game. It's all he ever uses anymore, and I need to find a way to get rid of it, because whenever something is resistant to paralysis, he gets mad saying I made it so that he can't do anything in the fight. Is there some way I can get rid of it in a fair way? Maybe a monster, or spell?