Dark is one of four known elemental attributes of skills. Dark-attribute skills often come in several forms: as green slimy poison that weakens and damages enemies over time, as an orb of energy that sucks all matter into it, or as a force that stirs its enemies around like a hurricane.
Although Dark-attribute skills have their advantages in slowing down enemies that approach them, the sheer number of mobs with natural Dark Resist makes it a less viable element to rely on alone in battle.