Thorn Swarm is the first upgrade on the second path for the Druid in BTD6. It allows the Druid to throw eight thorns instead of only five. The thorns are still thrown at random angles, but the thorns are overall packed denser as the random angular spread remains unchanged.
Thorn Swarm increases the number of projectiles for the Druid from five thorns to eight thorns. By increasing the number of thorns, this increases the density of its shotgun-like attack. It can come in handy when paired with Hard Thorns for countering stronger early-game bloons, or can be added for increased damage versus early MOABs when paired with Upper Path 3.
Thorn Swarm is useful for hitting singular targets, but it is not very reliable against grouped bloons compared to Hard Thorns, which can pop up to two bloons per thorn.
This should be considered when crosspathing the Druid. Usually, in late game, MOAB-class bloons are the main bloon sent out, so it is recommended to buy Thorn Swarm as point damage.
Earlier game, Thorn Swarm with Hard Thorns is a pretty good option as an early game tower.
Transitioning from a Thorn Swarm into the bottom path upgrades is also a good strategy, as it is a good crosspath with bottom path druids.
Thorn Swarm synergizes well with damage-increasing support such as Berserker Brew due to its 1 damage, which results in a major increase in popping power.
Thorn Swarm as seen in the upgrades menu
Thorn Swarm upgrade icon in upgrades menu
Crosspathing in BTD6 (Top: Path 1, Bottom: Path 3)
Upgrade portrait Thorn Swarm
Upgrade icon for Thorn Swarm
Instamonkey icon for Thorn Swarm
Buying Thorn Swarm will cause Poplust's cape to reduce in wideness in addition to adding the spiked bracelets.