In Divinity: Original Sin 2, the Blackroot herb is surprisingly crucial, especially for the Miester's Ritual in Act Four. This ritual, guided by Miester Siva, requires a Ritual Bowl (crafted from a ritual bowl, bloody obsidian lancet, and Blackroot) to unlock the Hall of Echoes and the coveted Night Vision skill.
Initially, all ingredients are conveniently located in Siva's basement. However, subsequent rituals (performed after gaining each Source point) require you to gather the ingredients yourself. While finding a ritual bowl and obsidian lancet is usually straightforward, locating Blackroot can be a challenge. This guide helps you overcome that hurdle.
Cloisterwood, a dense forest northwest of Driftwood on Reaper's Coast, is your target. Once inside, use the left ALT key to highlight all interactive objects. You'll find Blackroot at the base of certain trees. Gather a few, even though you only need one for the ritual. This guide avoids spoiling the number of times the ritual is needed, but remember you can perform it after each Source point gain.
Cloisterwood offers more than just Blackroot. Explore to discover two NPCs (Hannag and Jahan) who can potentially grant Source points (Jahan is also relevant to Lohse's storyline), an undead trader (Eithne) offering a "Divine Order" side quest, and a challenging high-level witch.
Further north lies a pier and the Dead Ferryman, offering passage to Bloodmoon Island. Beyond these key NPCs, explore Wrecker's Cave and seek out the spirit of a ship captain for a potential loremaster amulet. New players should thoroughly explore Cloisterwood, ideally leaving the challenging witch, Alice Alisceon, until reaching at least level 15.