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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 access the Hall of Echoes and obtain the 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 find everything yourself. Locating the ritual bowl and obsidian lancet is usually straightforward, but the Blackroot can be elusive. This guide helps you track it down.
Venture Into Cloisterwood
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 Blackroots at the base of certain trees. Gather a few, even though you only need one per ritual. (This guide avoids specifics on the number of rituals required to prevent spoilers.)
Cloisterwood Exploration Tips
Cloisterwood offers more than just Blackroots. Explore to discover: two NPCs (Hannag and Jahan) who can each help you gain Source points (Jahan also advances Lohse's storyline); Eithne, an undead trader offering a "Divine Order" side quest; and a challenging witch (best avoided until level 15). Further exploration might reveal the Wrecker's Cave and a unique loremaster amulet. Thorough exploration is recommended on your first visit.