Unleashes regular blasts of forked lightning that arc from bloon to bloon.
~ BTD6 Description
Heart of Thunder is the 2nd upgrade of the 1st path in Bloons TD 6. It allows the Druid shoot lightning bolts similar to a 2-x Monkey Apprentice in BTD5 albeit with a much lower pop limit and attack speed. It is shot every 2.3 seconds and dealing 1 layer of damage and having 31 pierce. It also does follow along the path and when paths intersect and near each other, such as on Cubism, it will jump from path to path randomly zapping bloons. The bolt cannot pop Purple Bloons, although the main thorn attack still does.
Similarly to BTD5's Lightning Bolt from the Monkey Apprentice, Heart of Thunder is very useful at countering Lead Bloons and grouped bloons from far distances of the map.
This upgrade combined with Druid of Wrath can accelerate wrath bonuses very quickly compared with crosspathing with Thorn Swarm, due to how much more pierce Heart of Thunder can impact onto massive groups of regular bloons. Druid of Wrath's wrath bonuses are dependent on the amount of total damage dealt consecutively, assuming that the Druid of Wrath hasn't become idle within 2.0s.