Mindclaw Shaman

Any player can respond to Mindclaw Shaman’s triggered ability. However, once that ability starts resolving and the target opponent reveals their hand, players can’t respond until after the ability finishes resolving. For example, the opponent can’t cast the instant or sorcery card you choose to cast in order to stop you from casting it.

If you can’t cast any instant or sorcery card (perhaps because there are no legal targets available) or if you choose not to cast one, then Mindclaw Shaman’s ability finishes resolving and the game moves on.