Does a damaged unit die immediately when becoming defender on Forbidden Waste?

If combat starts normally, your unit dies immediately when it becomes a defender — Forbidden Waste is passive and does not use the chain, so you cannot react. If the attacker has a move trigger, combat cannot start until that resolves, giving a reaction window. You do not lose control during a showdown, so hidden cards remain, but your unit dies before you can Hidden Blade it. You can still target opponent units.