From BZPB Wiki

Death is the termination of the biological functions that sustain a living organism. The word refers both to the particular processes of life's cessation as well as to the condition or state of a formerly living body. Phenomena which commonly bring about death include predation, malnutrition, accidents resulting in terminal injury, and disease.

However, you may not be aware that death exists not merely as a state, but as an individual, who is also called Death.

Or so he likes people to think.

In reality, Death is merely a being with delusions of grandeur, who works for Blackout for some reason. However, since he is capable of killing people and/or bringing them back to life, some say he has a pretty good claim to be the actual personification of death.

Sometimes he works alone, sometimes it employs agents. One of these agents was Skelly Bones!, whom Death sent to get rid of Yon (SMU).

Later, Death was summoned by Blackout's Dark Knights during one of their many attacks against Aeturnus' men.