When your Planeswalker is injured you can heal them by waiting for their health to return over time or by using a potion. Potions heal more quickly (and your potion supply recharges over time). Potions are represented by a (+) below a Planeswalker's health when selecting a Planeswalker. It is recommended that players have multiple Planeswalkers so they can continue playing when one is injured.

To purchase a potion - 1 potion = 10 Mana Crystals and 3 potions = 25 Mana Crystals