Crazed Furbolg Blood is dropped by various Redfang and Frostpaw furbolgs. They can be found fighting near Grizzlemaw, as well as several other places. Some amount of the blood may be acquired while completing the quests
Note that slaying a member of one tribe will grant you a buff, "Redfang Ally" or "Frostpaw Ally", making the player neutral to that tribe for 5 minutes.
As a quest objective Edit
This item is required for the quests