Cruel Ultimatum’s only target is an opponent. You don’t choose which creature card in your graveyard you’ll return to your hand until Cruel Ultimatum resolves.
All of these actions are performed sequentially, in the order listed. Earlier actions may affect how you perform later actions. (For example, if the opponent sacrifices a creature that he or she controls but you own, it will end up in your graveyard. When Cruel Ultimatum lets you return a creature card from your graveyard to your hand, you can choose that one.)
If, as Cruel Ultimatum begins to resolve, your opponent’s life total is 5 or less and you have two or fewer cards in your library, the game will result in a draw. Your opponent’s life total will drop to 0 or less, but Cruel Ultimatum must finish resolving completely before state-based actions are checked. You’ll then be forced to draw three cards and fail to draw one. When state-based actions are finally checked, you and your opponent will both lose the game at the same time, which means the game is a draw.