Wildfire Devils — ruling 1

You choose a player at random and they exile a card all while the triggered ability is resolving. No player may take any actions between the two or between exiling the card and you copying and casting it.

Wildfire Devils — ruling 2

The chosen player chooses which card to exile from their graveyard.

Wildfire Devils — ruling 3

If the chosen player has no instant or sorcery cards in their graveyard, Wildfire Devils’s ability does nothing. You don’t pick a different player.

Wildfire Devils — ruling 4

The copy of the card is created in exile and cast from exile, not from a graveyard.

Wildfire Devils — ruling 5

You can cast the copy only as the triggered ability resolves. If you don’t want to cast it at that time (or you can’t cast it, perhaps because there are no legal targets available), the copy ceases to exist. You can’t cast it later.

Wildfire Devils — ruling 6

You can cast the copy during the resolution of the triggered ability if it’s a sorcery, no matter whose turn it is or which phase it is.

Wildfire Devils — ruling 7

If a spell has {X} in its mana cost, you must choose 0 as the value of X when casting it without paying its mana cost.

Wildfire Devils — ruling 8

If you cast a card “without paying its mana cost,” you can’t choose to cast it for any alternative costs. You can, however, pay additional costs. If the card has any mandatory additional costs, such as that of Momentous Fall, you must pay those to cast the card.

Wildfire Devils : règles, fonctionnement et interactions (MTG)

Wildfire Devils 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

When this creature enters and at the beginning of your upkeep, choose a player at random. That player exiles an instant or sorcery card from their graveyard. Copy that card. You may cast the copy without paying its mana cost.

How it works : Wildfire Devils

Original English text — Wizards of the Coast / Scryfall

#1August 23, 2019· Wizards of the Coast
You choose a player at random and they exile a card all while the triggered ability is resolving. No player may take any actions between the two or between exiling the card and you copying and casting it.
No other card is named in this ruling.
#2August 23, 2019· Wizards of the Coast
The chosen player chooses which card to exile from their graveyard.
No other card is named in this ruling.
#3August 23, 2019· Wizards of the Coast
If the chosen player has no instant or sorcery cards in their graveyard, Wildfire Devils’s ability does nothing. You don’t pick a different player.
No other card is named in this ruling.
#4August 23, 2019· Wizards of the Coast
The copy of the card is created in exile and cast from exile, not from a graveyard.
No other card is named in this ruling.
#5August 23, 2019· Wizards of the Coast
You can cast the copy only as the triggered ability resolves. If you don’t want to cast it at that time (or you can’t cast it, perhaps because there are no legal targets available), the copy ceases to exist. You can’t cast it later.
No other card is named in this ruling.
#6August 23, 2019· Wizards of the Coast
You can cast the copy during the resolution of the triggered ability if it’s a sorcery, no matter whose turn it is or which phase it is.
No other card is named in this ruling.
#7August 23, 2019· Wizards of the Coast
If a spell has in its mana cost, you must choose 0 as the value of X when casting it without paying its mana cost.
No other card is named in this ruling.
#8August 23, 2019· Wizards of the Coast
If you cast a card “without paying its mana cost,” you can’t choose to cast it for any alternative costs. You can, however, pay additional costs. If the card has any mandatory additional costs, such as that of Momentous Fall, you must pay those to cast the card.
No other card is named in this ruling.