"Enrage your units to make them bigger, faster and stronger! Cast this to create a Ring of Rage! Your units will gain speed and attack power while they are inside this ring."
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.
The Rage Spell increases each individual unit's damage by a fixed percentage (which increases as the spell's level rises). Because of this, the spell is most effective when using it with high-damage Troops such as P.E.K.K.As and Dragons.
Although they deal no damage, Healers can take advantage of the spell as well, as it will cause them to heal faster (and reduce the need for Healing Spells while the Rage Spell is active).
The most effective placement of the Rage Spell is to position it approximately 3-4 squares toward where your Troops' next target will be; this will give your Troops the maximum amount of time inside the Rage Spell's area of effect as they destroy their current target and move to the next one. If you drop it directly under their feet, they will quickly move out of its range once they have finished with their current target.
The Rage Spell is substantially less effective when combined with Tier 1 and Tier 2 troops only, as they don't do much damage as compared to the Tier 3 troops. Your primary target should be your highest damage troops, and any Tier 1 and Tier 2 troops it happens to affect is simply extra damage.
The P.E.K.K.A is especially well-suited for the Rage Spell, as she normally moves and attacks quite slowly.
The Rage Spell's effects do not stack, even if multiple rage spells are used on the same location.
The Healing Spell goes well with the Rage Spell, due to the fact that it makes Troops faster and stronger yet healing them, making them much more durable.
Although the Rage Spell can be upgraded to level 2 in the level 3 Laboratory, only Troops and Spells that are already unlocked will appear in the Laboratory upgrade interface. The Rage Spell will not appear in the Laboratory until the Spell Factory has been upgraded to level 3 (which requires Town Hall level 7). Most players already have a level 5 Laboratory by that time; if so, the first three upgrades will be available as soon as the Spell Factory level 3 upgrade finishes.