Antipodal Amulet

A plain-looking circular wrought-iron amulet with a pattern of twisted metal in the middle.

Once per long rest, the wearer of this amulet may use it when casting a spell. The spell will have the opposite of its normal effect. What this means is up to the DM's discretion, although the player may explain what effect they had in mind. For example it might change a fire spell to a water or ice spell, thus changing its damage type. The spell Create Bonfire could be used to magically remove an existing bonfire. Perhaps Expeditious Retreat could be cast on an opponent, preventing them from taking the dash action for the duration.

The possibilities are endless and open up a lot of creativity for the player and the DM to work together on how to resolve specific uses.