From
Star Trek:
Energy reactors that were converted by the Menthars into weapons. They were modified so that they would absorb energy from nearly energy sources and turn it into deathly radiation. They were placed in booby trap configuration in the asteroid field left over from the planet Orelious 9.
The booby trap trapped a Promellian battlecruiser around 2266 and killed the crew. In 2366 the curious Enterprise-D came to investigate the cruiser which was almost perfect condition, but also became trapped in the booby trap. They were able to get out because the assimilators were so old that they wouldn't respond to either very slow or very rapid action.
See: Booby Trap (TNG)