Target opponent sacrifices a creature, discards three cards, then loses 5 life. You return a creature card from your graveyard to your hand, draw three cards, then gain 5 life.
3/14/2017 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.
3/14/2017 All of the actions for Cruel Ultimatum 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 they control 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.
3/14/2017 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 performed. You’ll then be forced to draw three cards and fail to draw one. When state-based actions are finally performed, you and your opponent will both lose the game at the same time, which means the game is a draw.