Vestcake's Floating Curse

Mentioned in The Colour of Magic, Vestcake's Floating Curse was described as a reasonably powerful spell. Anyone who saw someone about to cast it would have promptly run away.

The Arch-Astronomer of Krull, when he realized that the two chelonauts going to board the spacecraft Potent Voyager were imposters (specifically, they were Rincewind and Twoflower) prepared to fire this spell at them before they boarded the ship and escaped. However, he faltered in the middle of casting the spell when he spotted a sinister dome of seaweed that had broken down the gates of the arena and began attacking everything in sight. He chose to use the more powerful Infernal Combustion Enigma instead, to try and destroy it. However, that didn't work either...