In Divinity: Original Sin 2, the Blackroot herb is surprisingly crucial, especially for the Miester's Ritual in Act Four. This ritual, overseen 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.
While the initial ritual ingredients are conveniently located in Siva's basement, subsequent rituals (performed after gaining each Source point) require you to gather everything yourself. Finding the ritual bowl and obsidian lancet is usually straightforward, but the Blackroot can be elusive. This guide helps you locate it.
Cloisterwood, a dense forest northwest of Driftwood on Reaper's Coast, is your target. Once inside, use the ALT key to highlight all interactable objects. You'll find Blackroot at the base of certain trees. Gather a few, even though you only need one per ritual. (This guide avoids spoiling the number of rituals needed, but remember, each Source point earns you another chance.)
Cloisterwood offers more than just Blackroot. Explore to discover two NPCs (Hannag and Jahan—the latter crucial for Lohse's storyline) who can each grant a Source point. You'll also find Eithne, an undead trader in the ruins; interact with her and choose the "Divine Order" option for a side quest.
Further north lies a pier and the Dead Ferryman, offering passage to Bloodmoon Island. Beyond these NPCs, Cloisterwood holds other secrets: Wrecker's Cave and potentially a unique loremaster amulet (from a ship captain's spirit). Newcomers should explore thoroughly, but consider avoiding the challenging witch, Alice Alisceon, until reaching at least level 15.