You gain Dark Essence by defeating monsters. Dark Essence resets at the beginning of each new round of combat. You use Dark Essence for upgrades. You gain Light Essence by collecting light orbs. The first light orb each round appears next to you. After collecting a light orb, another appears randomly around the area after 5-ish seconds. You use Light Essence to summon more turret-like allies called Souls. Light Essence also lights up the arena for a period of time. You go around collecting the light orbs, slowly creating more Souls, and getting upgrades.