Gains 10% attack speed and 1% for every life lost from the starting value after the upgrade. Maxes at 100% and diminishes if lives are restored.
~ BTD6 official description, as of Version 17.0
Heart of Vengeance is the second upgrade of Path 3 for the Druid. It gives the Druid automatic 10% attack speed, and afterwards adds a 1% attack speed buff for each life lost from when the upgrade is bought. The attack speed buff is capped at 100%, meaning that the maximum attack speed buff is attained when 90 lives are lost. This buff will reverse if lives are gained. This lives lost does not consider any lives above the original amount of lives.
It costs $255 on Easy, $300 on Medium, $325 on Hard, and $360 on Impoppable.
Just as losing lives will increase attack speed, gaining lives will make Druids with this upgrade attack slower. It is best not to use life gaining strategies with this path of upgrades, such as with Benjamin, Support Chinook's Supply Crate ability, or the Spirit of the Forest upgrade.
However, in practice, this effect is not as pronounced as the following upgrades can still be functional without the attack speed buff.
Lives absorbed by the Mana Shield do not count towards the attack boost.
Note that using a Continue will decrease Heart of Vengeance bonuses, as using a Continue gains lives, thus lowering Heart of Vengeance bonus.
Heart of Vengeance used to be useless on CHIMPS Mode, but has since changed after Version 9.0 by providing +10% bonus attack speed by default.
Before Version 17.0, the description was "Gains attack speed up to 100 lives lost after getting this upgrade, Lives above maximum do not count." but was changed to clarify the new special life-loss mechanic that was introduced from Version 9.0.