Fervor

Fervor is a stackable Buff that at maximum stacks provides the target with 50% bonus damage dealt in Spiritual damage. This seems to only occur on direct damage skills. For example, Explosive Trap will not deal bonus damage, but Volley will. Poisoned Arrow will deal 50% bonus damage on each tick, but the extra damage from movement will not get this bonus (needs confirmation). Fervor stacks additionally allow the Shaman to heal his target more effectively.

Fervor is given via the Spiritual Fervor skill, used by the Shaman. As expected, this is the only means of getting this Buff.