Booby trap

A booby trap was a secreted device that would attack those who triggered it.

Knowing there was a chance Eldrad would return to Kastria, the Kastrians placed booby traps in their subterranean dwelling.

When Eldrad did come back, after one hundred and fifty million years, a door he opened triggered a booby trap. A dart containing an acid that neutralised the molecular bond hit him. As there was no antidote, the acid being of Eldrad's own design, he asked the Fourth Doctor to deliver him to a regenerator chamber.

The door of the chamber was also fitted with a booby trap, which the Doctor detonated with no ill effects. (TV: The Hand of Fear)