I think, you are mistaking it for another NPC with such name: Thorn Tentacle

I have just obtained the knowledge of "Toxic Thorn Tentacle" by killing NPC "Toxic Thorn Tentacle" in the Northern Swamp, which is just south of Heidel. The two NPCs "Thorn Tentacle" and the "Toxic" variant are both there. On map, go due south from Heidel storage keeper, across the river to the body of water that is Southeast of the small farm.

ETA: the map markers provided by the codex are correct.