If Seraph isn’t on the battlefield at the time the other creature is put into a graveyard, the ability won’t trigger. However, the ability does trigger if Seraph and the other creature are put into the graveyard at the same time.

If the other creature is a token, it will cease to exist after being exiled. It won’t be returned to the battlefield.

If the other creature ceases to be a creature card after it leaves the battlefield (it’s an animated Mishra’s Factory, for example), it will be returned to the battlefield by this ability and you will have to sacrifice it when you lose control of Seraph.

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.

You do not get the creature back if the creature is not still in the graveyard at the end of the turn.

Seraph : règles, fonctionnement et interactions (MTG)

Seraph is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

Flying Whenever a creature dealt damage by this creature this turn dies, put that card onto the battlefield under your control at the beginning of the next end step. Sacrifice the creature when you lose control of this creature.

How it works : Seraph

Original English text — Wizards of the Coast / Scryfall

#1September 16, 2007· Wizards of the Coast
If Seraph isn’t on the battlefield at the time the other creature is put into a graveyard, the ability won’t trigger. However, the ability does trigger if Seraph and the other creature are put into the graveyard at the same time.
No other card is named in this ruling.
#2September 16, 2007· Wizards of the Coast
If the other creature is a token, it will cease to exist after being exiled. It won’t be returned to the battlefield.
No other card is named in this ruling.
#3September 16, 2007· Wizards of the Coast
If the other creature ceases to be a creature card after it leaves the battlefield (it’s an animated Mishra’s Factory, for example), it will be returned to the battlefield by this ability and you will have to sacrifice it when you lose control of Seraph.
No other card is named in this ruling.
#4September 16, 2007· Wizards of the Coast
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.
No other card is named in this ruling.
#5October 4, 2004· Wizards of the Coast
You do not get the creature back if the creature is not still in the graveyard at the end of the turn.
No other card is named in this ruling.

Interactions with other cards

These cards' text mentions Seraph.

Breathkeeper Seraph

Flying, soulbond (You may pair this creature with another unpaired creature when either enters. They remain paired for as long as you control both of them.) As long as Breathkeeper Seraph is paired with another creature, each of those creatures has "When this creature dies, you may return it to the battlefield under its owner's control at the beginning of your next upkeep."

Seraph of the Masses

Convoke (Your creatures can help cast this spell. Each creature you tap while casting this spell pays for {1} or one mana of that creature's color.) Flying Seraph of the Masses's power and toughness are each equal to the number of creatures you control.