If opponent plays Icathian Rain and chooses to ping my unit twice, and I respond with Hidden Blade to kill my own unit, does opponent get to choose new targets or does the target become invalid?

Ruling: Once Icathian Rain's reflexive triggers are put on the chain with targets chosen, your opponent cannot choose new targets. If you kill the targeted unit with Hidden Blade before the damage resolves, those damage instances will simply do nothing.

Sequence:

• Opponent plays Icathian Rain (does not target initially)
• Icathian Rain resolves and creates 6 reflexive triggers of 2 damage each
• Opponent chooses targets for each reflexive trigger
• All 6 triggers are put on the chain with their chosen targets
• You can respond with Hidden Blade before any damage instance resolves
• If Hidden Blade kills the targeted unit, the damage instances targeting it will do nothing

Nuances:

• Icathian Rain itself does not target; it creates reflexive triggers that do target
• You cannot select new targets for spells once they are put on the chain
• Fox Fire is an exception, but it doesn't choose new targets - it just chooses a subset of already-chosen targets