Mana Cache

At the beginning of each player's end step, put a charge counter on this enchantment for each untapped land that player controls. Remove a charge counter from this enchantment: Add {C}. Any player may activate this ability but only during their turn before the end step.

From Magic: The Gathering.

Open the interactive version on https://mythscribe.cloud/app/tcg/cards/cms6obl6j7vmppb1hpqgpe4j0.