Once the ability triggers, you’ll return the card to the battlefield under your control at the end of the turn even if you no longer control Seraph at that time. In fact, if Seraph leaves the battlefield before the card is returned to the battlefield, you’ll never have to sacrifice it as a result of this ability. If Seraph remains on the battlefield but changes controllers before the card is returned, you’ll still get the returned card. However, in that scenario, you’ll still have to sacrifice it if you later regain control Seraph and then lose control of it once more.