The Deathless

Little is known about the deathless - they are a group of assassins that were known for their cruel efficiency, impossible accomplishments and surgical precision with death. If a king ended up dead locked inside his own vault, it was assumed to be the deathless. Through many ages they were considered to be the god of death himself though there are two recorded reports of actual evidence to suggest they exist aside from the obvious results of their work.

Records
One wizard managed to accidentally kill a deathless while performing magical experimentation with no knowledge of the deathless nearby. He found the tattered clothing and a metal mask in the shape of an angular skull and immediately spread news far and wide that he had not just SEEN a deathless, but killed one. The wizard died the very next day, the angular skull mask disappeared, but drawings of it spread across the realm faster than wildfire.

A Noble drunkenly boasted once that he had hired the deathless to kill a rival. He claimed that the deathless operate on an 'oath pact' where the deathless will only commit to killing a single target at a time, and they will pursue no other until the deed is done. The pact that is made (after incredible amounts of gold is exchanged) gives them power to carry out their task. This would prevent them from carrying out mass murder or being used as an mercenary army, but a leader or general could be assassinated, though surprisingly there are no recorded instances of deathless assassinations during war.

Disappearance
At the height of their popularity as a tool for murder, one day the deathless assassinations just stopped. The deathless passed out of knowledge and became myth. Assassinations still occurred but they were imperfect and carried out by regular criminals and killers, no one knows why. It is rumored that the deathless entered into a pact to kill someone that they have been unable to carry out for the past few hundred years, though no one thought that possible.