A Death universe is a normal Universe, whose laws have been set, either naturally or artificially, in such a way, that when anything enters in it, it ceases to live.
These Death universes do not actually exist, they are merely thought experiments, as having a Universe cease any life whatsoever that enters it, is impossible, due to how technologically advanced some creatures are, and even ones at the lower spectrum of this scale could withstand many such verses.
The Death universe's counterpart, the Life universe, is a verse that when collided with the Death universe, produces a "normal universe". This definition is very arbitrary, and highlights the whole fact about this being just a thought experiment.
Equivalents, but not specifically Death universes, include any verses which one could consider having extreme properties, like extreme Eisler values, or extreme temperature, or black holes, for example.