Basilisk Oil is a "colorless potion" found in Ultima Underworld II. It is rare, but only useful for the purpose of changing the Filanium Mud of Anodunos into something needed for the Djinn Bottle. As such it is a plot critical item and it's impossible to finish the game if, for example, the player drinks all three potions without realising what it's for.
The three known locations are:
- Sewers of Britannia, guarded by two Gazers.
- Pits of Carnage, Zaria's chest.
- Pits of Carnage, secret room of the Lich.
- Swallowing the oil is the best way to give the Avatar a good drug trip, as it has really interesting results on the eyes.