Suppose my opponent has a Spellbender secret and a minion with at least 5 attack. I cast Shadow Word: Death on the big minion. Spellbender is supposed to summon a 1/3 minion as the new target, but Shadow Word: Death can only target a minion with 5 or more attack.
Does the new minion get destroyed, does the spell fail, or does something else entirely happen?