Where to find Blackroot plant - Divinity: Orignal Sin 2

After you obtain your second source slot and you have done the trial at Meistr Siva, you will need to unlock Source Vampirism. This will allow you to consume wandering souls and bodies. For that you will need to go to Hall of Echoes once again and to the fire breathing ritual. The important ingredient here is Blackroot. Which you can find Cloisterwood near undead Trader Eithne:

The coordinates are: x - 158, y - 221