Hellfire Mongrel — ruling 1

Hellfire Mongrel’s triggered ability has an “intervening ‘if’ clause.” That means (1) the ability won’t trigger at all unless the opponent whose turn it is has two or fewer cards in hand as their upkeep starts, and (2) the ability will do nothing if that player has three or more cards in hand by the time it resolves.

Hellfire Mongrel — ruling 2

In a Two-Headed Giant game, this ability will potentially trigger twice at the beginning of the opposing team’s upkeep — once for each player on that team.

Hellfire Mongrel : règles, fonctionnement et interactions (MTG)

Hellfire Mongrel 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

At the beginning of each opponent's upkeep, if that player has two or fewer cards in hand, this creature deals 2 damage to that player.

How it works : Hellfire Mongrel

Original English text — Wizards of the Coast / Scryfall

#1October 1, 2009· Wizards of the Coast
Hellfire Mongrel’s triggered ability has an “intervening ‘if’ clause.” That means (1) the ability won’t trigger at all unless the opponent whose turn it is has two or fewer cards in hand as their upkeep starts, and (2) the ability will do nothing if that player has three or more cards in hand by the time it resolves.
No other card is named in this ruling.
#2October 1, 2009· Wizards of the Coast
In a Two-Headed Giant game, this ability will potentially trigger twice at the beginning of the opposing team’s upkeep — once for each player on that team.
No other card is named in this ruling.