Nourishing Shoal

Instant — Arcane

You may exile a green card with mana value X from your hand rather than pay this spell's mana cost.
You gain X life.

2018-12-07:
If a card in a player’s hand has {X} in its mana cost, X is considered to be 0.
2018-12-07:
If a spell has {X} in its mana cost, include the value chosen for that X when determining the converted mana cost of that spell, even if it was cast for an alternative cost and no mana was spent on X.