MTG Treachery

The Reflector’s unveiling triggered ability can target any instant or sorcery spell, or ability on the stack, even one without targets. Static abilities of objects can’t be targeted.
If you gain control of an instant or sorcery spell, it will be put into its owner’s graveyard as it resolves.
If you gain control of an instant or sorcery spell for which a Buyback cost has been paid, the card returns to its owner’s hand.

You may change any or none of the spell’s targets. If you change a target, you must choose a legal target for the spell. If you can’t, you must leave the target unchanged (even if the current target is illegal). Notably, if you were originally chosen as a "target opponent" for a spell you gain control with The Reflector, you are now an illegal target as you aren’t your own opponent.
If the target is removed from the stack (e.g. countered) before the unveiling ability resolves, The Reflector will fizzle and do nothing as it is its only target.