If Leyline of Lightning 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, Leyline of Lightning deals 1 damage to target player or planeswalker.
2/1/2006 You may pay the only once per spell.
2/1/2006 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 the battlefield with counters from its bloodthirst ability.
6/8/2016 A player’s “opening hand” is the hand of cards the player has after all players have taken mulligans and “scryed” if applicable. 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.