Devotion measures the colored mana symbols in the mana costs of permanents you control. Cards can care about devotion to one or more colors, rewarding decks that heavily commit their permanents to a particular slice of the color pie.
Master of Waves creates more Elementals as your devotion to blue increases, while Gods such as Heliod and Thassa require sufficient devotion before they can become creatures on the battlefield. Nykthos can turn a large devotion count into a torrent of mana.
