Heart of Thunder

"Unleashes regular blasts of forked lightning that arc from bloon to bloon."

- In-game description

 is the second upgrade of the top path for the Druid in Bloons TD 6. It allows the Druid to shoot lightning bolts, similar to a 2-x Monkey Apprentice in BTD5. Lightning bolts are shot every 2.3 seconds, deal 1 layer of damage and have 31 pierce per bolt. These bolts also follow along the track; and when paths intersect or are near to each other, such as on Cubism, it will jump from path to path randomly zapping bloons. Lightning seems to always target bloons behind its initial target, and virtually never arcs towards bloons ahead of it.

Although the Druid's thorns keep the Normal damage type, bolts cannot affect Purple Bloons.

It costs $850 on Easy, $1,000 on Medium, $1,080 on Hard, and $1,200 on Impoppable.

Description
Adding this upgrade grants the Druid new blue headwear, as well as a carved diamond on top of its wooden staff.

Purchasing the Heart of Thunder upgrade grants the Druid a new lightning attack. While the seeking AI of the lightning bolt is not fully known, on certain maps it bounces to the last 30 bloons on the screen. This means the lightning bolts actually have 31 pierce, since the lightning bolt attack sparks from one bloon and jumps to up to 30 other bloons.

Summary
Heart of Thunder works very similarly to the Monkey Apprentice's Lightning Bolt upgrade, and therefore can be used to counter large groups of bloons due to the nature and pierce of the forked lightning. Since a single lightning bolt spawning numerous Pinks can overwhelm Druid, a good handful of Druids with Heart of Thunder may be necessary to optimize the grouped damage potential of this new attack. Druids can be upgraded with Heart of Oak to strip Regrow properties or gain further attack speed along Path 3 (e.g. Heart of Vengeance) to boost their grouped bloon popping potential.

Tips

 * 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.
 * This upgrade, combined with Druid of Wrath, can accelerate wrath bonuses very quickly compared to Thorn Swarm crosspaths, due to how much the pierce Heart of Thunder provides can impact large groups of bloons. Druid of Wrath's wrath bonuses are dependent on the amount of total damage dealt consecutively, assuming that the Druid of Wrath is not idle for more than 2.0s.
 * Note that the lightning bolts cannot affect Purple Bloons. There are alternative upgrades available that act similarly to Heart of Thunder and can hit Purple Bloons, including Boomerang Monkey's Glaive Ricochet and M.O.A.R. Glaives upgrades.
 * The 2-2-0 Heart of Thunder is a decent choice for removing Regrow properties off grouped Regrow Bloons, such as those on Rounds 76 or 79, due to the Heart of Oak buff in Version 18.0.

Sounds
When striking lightning:
 * [electric zap]
 * [electric zap]
 * [electric zap]
 * [electric zap]
 * [electric zap]

Trivia

 * As of Version 18.0, a 2-2-0 Druid is roughly equivalent to a Rank 4+ (from Monkey Knowledge Packs) 2-0 Monkey Apprentice on Bloons Monkey City, except lightning bolts are guaranteed to remove Regrow properties instead of being capped at a 70% chance.
 * According to Ninja Kiwi's 5th November 2021 blog, Druid's lightning is based on "seeded formulaics" rather than pure RNG.