Killing Bow

In storytelling, a plot element set-up for future payoff is known as “Chekov’s Gun”. The idea being that you don’t introduce a gun in Act I if you don’t intent to fire it by Act III. Magic items in D&D can be similar. As a DM, you shouldn’t introduce a cool magic item that you don’t want to be used or abused. And sometimes it’s fun to set-up a future encounter via some neat treasure. But this can’t always be planned for, and the players really might forget that they have the perfect magic item in a situation until the moment has passed.