Say that player A has two heroes and Ark of Salvation in play. A attacks player B with one hero, leaving the other in territory. B blocks with Assyrian Archer banded to Assyrian Siege Army.
Assyrian Archer: You may discard a hero in a territory. If you do, discard this card after battle. May band to an Assyrian.
Archer attempts to discard A's hero in territory, but cannot because it is protected by Ark of Salvation. However, then Siege Army activates, negating and discarding Ark. Does Archer then discard the hero in territory, or does the protection mean that it isn't a valid target in the first place, and Archer just doesn't have a target?