Wake to Slaughter

You can cast a spell using flashback even if it was somehow put into your graveyard without having been cast.

To determine the total cost of a spell, start with the mana cost or alternative cost (such as a flashback cost) you're paying, add any cost increases, then apply any cost reductions. The mana value of the spell is determined only by its mana cost, no matter what the total cost to cast the spell was.

If only one target is still legal as Wake to Slaughter resolves (or if you only chose one target), that's the card your opponent will have to choose to put into your hand. They can't choose to have you return it to the battlefield.