You get it from your God after doing second Ritual at Meistr Siva house.
First ritual is ability to see Spirits, second ritual gives you Source Vampirism (you do everything the same, but you need Blackroot which no longer is free for you, you need to take one near Cloisterwood near Eithne)
http://divinityoriginalsin2.wiki.fextralife.com/file/Divinity-Original-Sin-2/Blackroot.jpg
here is Blackroot cords to pick up.