Goblin Fire Fiend — ruling 1

If Goblin Fire Fiend is attacking, the defending player must assign at least one blocker to it during the declare blockers step if that player controls any creatures that could block Goblin Fire Fiend.

Goblin Fire Fiend : règles, fonctionnement et interactions (MTG)

Goblin Fire Fiend 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

Haste This creature must be blocked if able. : This creature gets +1/+0 until end of turn.

How it works : Goblin Fire Fiend

Original English text — Wizards of the Coast / Scryfall

#1October 1, 2005· Wizards of the Coast
If Goblin Fire Fiend is attacking, the defending player must assign at least one blocker to it during the declare blockers step if that player controls any creatures that could block Goblin Fire Fiend.
No other card is named in this ruling.