Reapers

Reapers are the guardians of the dead. They take the souls of the departed to the Underworld, and protect them from those who would wish to abuse them.

They're also ridiculously mysterious. They tend to be just about as aloof and uninvolved in the politics of the supernatural world as angels, generally only getting worked up if someone's doing something nasty with the cycle of life and death (genocide, trying to create a species, attempting an immortality ritual, general undead mischief, the like). Now, there are some exceptions, and some reapers do muddle about in the other realms, but to a last reapers do not talk about their kind, or how they became a reaper. The only thing that is known is that every reaper has so far claimed to a member of one of the other besouled races before death (aside from horrors, whose souls are doomed to be destroyed in the Pit when they die, and angels... for some reason.) Some reapers do mention making a deal with death to become reapons, but it's not known if this is figurative or if Death is an actual entity.

Likewise, it appears that even reapers don't know what happens to souls after they reach the Underworld. Some scholars believe the souls are recycled, and reborn into the mortal world. Others believe the souls wait there until they are called upon for a final judgement which will determine their true afterlife. Others disagree, and say that the judgement will only come during the end times... whatever those are. Still others argue that the underworld is the only thing there is, and that if one looks hard enough, one might be able to find the soul of any mortal who ever lived. Of course, given how the reapers don't exactly enjoy their own kind poking around the secret pockets of the Underworld, to say nothing of outsiders, these theories are kinda hard to prove.