MTG Treachery

The Spellsnatcher’s unveiling triggered ability can target any spell, even one without targets.
If you gain control of an instant or sorcery spell, it will be put into its owner’s graveyard as it resolves.
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 Spellsnatcher, you are now an illegal target as you aren’t your own opponent.

Combat damage dealt by a commander is tracked without regard to who controlled the commander at the time it dealt damage. For example, say a commander deals 10 combat damage to a player, leaves the battlefield, is recast, and you gain control of it with The Spellsnatcher. If it deals another 11 combat damage to that same player, they will lose the game.
If you gain control of a commander spell, the commander’s owner chooses whether to put it in the command zone if it later leaves the battlefield.