The Rage Spell creates a translucent purple ring on the battlefield that boosts the movement speed and damage of any friendly units inside the ring. In the case of Healers, the Rage Spell increases their healing by a similar amount.
There has been a great deal of confusion regarding whether the 'damage increase' listed is additive (+130% damage) or multiplicative (damage * 1.3). After multiple independent tests, it has been confirmed that the increase is indeed additive, meaning that troops under the influence of a level 1 Rage Spell do 130% more damage than those that are not; i.e. their damage (D) is calculated as follows: