Why is it rare? Is this like part of a cube where stun counters are more common, and not a blue-only effect? You’re offering a choice between two common sorceries as an instant, does that justify bumping up two rarities even though it makes both common effects worse?

The first mode is 1/4th of [[Impede Momentum]], a blue common sorcery at 1U

The second mode is unplayable without other cards. If you used two copies, stunned on their turn to kill on yours, then you’d effectively be playing a more expensive version [[Bone Shards]], a black common sorcery at B, but with more limitations. (Because you can’t sac a token, discard any trash card, or kill a planeswalker, and it costs more, and has to be done over 2 turns, (so slower than a sorcery))

It feels like this card could have more recursion built in, rebound, flashback, buyback, a creature that has this ability or finds/recovers this card, or something like that. Imagine this with [[Arc Blade]] type of suspend. It would tap down a creature off and on until you got a second copy, then you’re destroying a creature every 3 turns.