Leyline of Lightning — ruling 1

While resolving Leyline of Lightning's second ability, you can't pay {1} multiple times to deal damage multiple times.

Leyline of Lightning — ruling 2

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 Lightning — ruling 3

If you cast a creature with bloodthirst while Leyline of Lightning is on the battlefield, you may use the Leyline's ability to deal damage to an opponent. If you do, that damage is dealt before the creature spell resolves, and the creature will enter with counters from its bloodthirst ability.

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

Leyline of Lightning 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 cast a spell, you may pay . If you do, this enchantment deals 1 damage to target player or planeswalker.

How it works : Leyline of Lightning

Original English text — Wizards of the Coast / Scryfall

#1September 1, 2023· Wizards of the Coast
While resolving Leyline of Lightning's second ability, you can't pay multiple times to deal damage multiple times.
No other card is named in this ruling.
#2March 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.
#3February 1, 2006· Wizards of the Coast
If you cast a creature with bloodthirst while Leyline of Lightning is on the battlefield, you may use the Leyline's ability to deal damage to an opponent. If you do, that damage is dealt before the creature spell resolves, and the creature will enter with counters from its bloodthirst ability.
No other card is named in this ruling.