Elder Brain Blood is a quest item needed Baldur's Gate II: Shadows of Amn and Baldur's Gate II: Enhanced Edition during chapter 4. The Matron Mother needs this, or Kuo-toa's Blood or the eye of an Elder Beholder for the ritual. It can be acquired from the Elder Brain, which can be found in the Mind Flayer Dungeon which can be entered from the eastern part in the Underdark area.
In-game description
| “ | This is the blood of an Elder Brain. Squishy brain matter floats in the dark bloody substance. | ” |