A throwaway reference to a fictional machine in Vernor Vinge's A Fire Upon the Deep and The Cookie Monster.

Death cubes are not described explicitly, but from context they appear to be computing devices designed to
repeatedly run simulations of an uploaded consciousness, usually of human-equivalent intelligence.

One moral issue with this sort of application arise from fact that uploads in question are normally erased at the end of simulation. Death cube use would thus effectively be multiple homicide.

Another issue is that of suffering: in a manner similar to animal testing, but infinitely reproducible, a death cube could be used to test human reactions to millions of different painful or lethal scenarios.

A similar technology is implied in Survivor, the last part of Charles Stross' Accelerando.

Richard Morgan uses a death cube like technology as a torture device in his books.