Basilisk Oil

Basilisk Oil is a magical compound which appears in  Ultima Underworld II.

Description
To the untrained eye, basilisk oil appears to be a clear-colored potion, similar to the variety commonly used to restore spellcasters' mana. In reality, it is a potent hallucinogen which may be used in ritual magic, in particular, the binding of an air djinn. When seeking to shatter the Blackrock Dome which had enveloped Castle Britannia, the Avatar would have need of basilisk oil, which, mixed with the filanium mud of Anodunos, sealed such a djinn into the Avatar's body after further treatment through lava and the spell Iron Flesh. 

Locations
Basilisk oil may be found at the following locations in the course of Ultima Underworld II:
 * Castle Britannia, Level 2 - one container may be found in a cache guarded by two gazers.
 * Scintillus Academy, Secure Vault - in a chest in the outer vault.
 * Pits of Carnage, Level 1 - one container may be found in Zaria's quarters.
 * Pits of Carnage, Level 3 - three containers may be found in the secret quarters of a liche.

Trivia

 * It is very easy to waste all basilisk oil available and thus create an unwinnable situation, as one can't identify it with the low Lore skill available early in the game, and the bottles look exactly like potions of mana regain.
 * Actually drinking the oil causes interesting visual effects.