If I play Cull the Weak and I target my unit but my opponent reacts with Gust on my unit do I still kill the target that was gusted?
No, you do not kill the unit targeted by Cull the Weak if it is returned to your hand by Gust.
According to the rules of targeting and resolution (Rule 359.3.e.7 and RiftJudge FAQ #204), when Cull the Weak attempts to resolve, it checks for a legal target. Since your unit is no longer on the battlefield (it is back in your hand), it is no longer a valid target for the "kill one of their units" instruction.
The Sequence of Events:
- Playing Cull the Weak: You play the spell, designating your unit to be killed as your choice for the "Each player kills one of their units" instruction.
- Opponent's Reaction: Your opponent plays Gust in response, targeting that same unit.
- Chain Resolution (LIFO - Last-In, First-Out):
- Resolution of Gust: Gust resolves first, returning your unit to your hand.
- Resolution of Cull the Weak: When Cull the Weak attempts to resolve, it sees that the unit you initially chose is no longer on the board. Therefore, that specific part of the instruction cannot be executed for you.
The Outcome:
- Your side: Because your unit is no longer on the board, you cannot fulfill the instruction to "kill one of your units," and that part of the spell fails to execute for you.
- Opponent's side: Your opponent must still fulfill their part of the instruction. Even if you have no units remaining, they are still required to kill one of their own units to satisfy Cull the Weak.