Gains attack speed during the round as long as the Druid is popping Bloons.
Druid of Wrath is the third upgrade of Path 3 for the Druid. It allows the Druid to gain attack speed (0.5% more for each unit of damage it does during a round, up to 100%, resets after 2 seconds of being idle) the more bloons it has popped during the round. This attack speed bonus is stacked multiplicatively with attack speed bonuses from Heart of Vengeance, at maximum attack speed it reaches four times the base tower. It costs $510 on Easy, $600 on Medium, $650 on Hard, and $720 on Impoppable.
Level 18+ Obyn Greenfoot mentions Druids of Wrath pops starting "each round at 200", meaning that Druids of Wrath by default reach maximum wrath immediately when the round starts until after 2 seconds of being idle.