Underworld Cerberus — ruling 1

If you can't exile Underworld Cerberus when its last ability resolves, perhaps because it's been exiled by another spell or ability, each player will still return all creature cards from their graveyard to their hand.

Underworld Cerberus — ruling 2

If two of these die at the same time, the last ability of each will trigger. If each player controlled one, the nonactive player's ability will resolve first. The Underworld Cerberus controlled by that player will be exiled, and all other creature cards in graveyards (including the Underworld Cerberus controlled by the active player) will be returned to their owners' hands. Then the active player's ability will resolve. That Underworld Cerberus won't be exiled, but any creature cards in graveyards at that time, perhaps because they died in response to this ability, will be returned to their owners' hands.

Underworld Cerberus : règles, fonctionnement et interactions (MTG)

Underworld Cerberus 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

This creature can't be blocked except by three or more creatures. Cards in graveyards can't be the targets of spells or abilities. When this creature dies, exile it and each player returns all creature cards from their graveyard to their hand.

How it works : Underworld Cerberus

Original English text — Wizards of the Coast / Scryfall

#1September 15, 2013· Wizards of the Coast
If you can't exile Underworld Cerberus when its last ability resolves, perhaps because it's been exiled by another spell or ability, each player will still return all creature cards from their graveyard to their hand.
No other card is named in this ruling.
#2September 15, 2013· Wizards of the Coast
If two of these die at the same time, the last ability of each will trigger. If each player controlled one, the nonactive player's ability will resolve first. The Underworld Cerberus controlled by that player will be exiled, and all other creature cards in graveyards (including the Underworld Cerberus controlled by the active player) will be returned to their owners' hands. Then the active player's ability will resolve. That Underworld Cerberus won't be exiled, but any creature cards in graveyards at that time, perhaps because they died in response to this ability, will be returned to their owners' hands.
No other card is named in this ruling.