Leyline of Combustion — ruling 1

A player's "opening hand" is the hand of cards the player has after all players have taken mulligans. If players have any cards in hand that allow actions to be taken with them from a player's opening hand, the starting player takes all such actions first in any order, followed by each other player in turn order. Then the first turn begins.

Leyline of Combustion — ruling 2

Leyline of Combustion's triggered ability resolves before the spell that caused it to trigger. It resolves even if that spell is countered.

Leyline of Combustion — ruling 3

If a spell targets you and/or a permanent you control multiple times, or if a spell targets you and one or more permanents you control, Leyline of Combustion's triggered ability triggers once.

Leyline of Combustion : règles, fonctionnement et interactions (MTG)

Leyline of Combustion 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

If this card is in your opening hand, you may begin the game with it on the battlefield. Whenever you and/or at least one permanent you control becomes the target of a spell or ability an opponent controls, this enchantment deals 2 damage to that player.

How it works : Leyline of Combustion

Original English text — Wizards of the Coast / Scryfall

#1March 19, 2021· Wizards of the Coast
A player's "opening hand" is the hand of cards the player has after all players have taken mulligans. If players have any cards in hand that allow actions to be taken with them from a player's opening hand, the starting player takes all such actions first in any order, followed by each other player in turn order. Then the first turn begins.
No other card is named in this ruling.
#2July 12, 2019· Wizards of the Coast
Leyline of Combustion's triggered ability resolves before the spell that caused it to trigger. It resolves even if that spell is countered.
No other card is named in this ruling.
#3July 12, 2019· Wizards of the Coast
If a spell targets you and/or a permanent you control multiple times, or if a spell targets you and one or more permanents you control, Leyline of Combustion's triggered ability triggers once.
No other card is named in this ruling.