The strange potion is a quest item obtained during Zogre Flesh Eaters. After the player convinces Zavistic Rarve that Sithik Ints might be involved in the curse on Jiggig, Zavistic gives the player the strange potion with instructions to find some way to use the potion on Sithik and then question him.
The player must use the potion on the cup of tea in Sithik's room, then leave and return. The player will then find that Sithik has been transformed into an ogre. The potion can not be used on any other object or character.