The Headless Horseman

The Headless Horseman is a servant of Asmodeus who ordinarily dwells within the Realm of The Albtraum, although they are capable of leaving it.
== History ==
The horseman started life as a knight who led a life of war, until they found their soulmate - a beautiful Elven maiden. They laid down their arms and the two became lovers and were happy for a time, until the maiden was slain. Crying over her, the knight cursed the Heavens and and all the gods, swearing vengeance on her killers. They hunted down and killed all of the murderers, fighting through seemingly lethal wounds through sheer force of will, but were foiled in the end when one of their targets turned out to be an immortal spirit.
Asmodeus appeared before the knight, offering to give them the power they needed to complete their revenge in exchange for serving him "until an enemy takes [their] head". The knight accepted, and used the hellish powers granted by their new patron to obliterate the spirit so utterly that it could never again return from the Astral Sea.
From that point on, the fallen knight went out each nightfall, committing attrocities in Asmodeus's name, while attempting to do good deeds during the day in an attempt to assuage their conscience. During this time they even managed to make a friend. However, Asmodeus took advantage of this, sending a servant in mortal guise to reveal the rider's double life to the friend, who took up arms to stop them.
The rider easily won the confrontation, but could not bring themself to end their only friend's lide, and the friend took advantage of this brief lapse in focus to behead the rider. As their killer did not technically count as an enemy, this was not enough to complete the knight's bargain and the servant took the head away with them, and with it the horseman's last semblance of humanity.
Given that the horseman's head is now in the Realm of the Albtraum and that the horseman also spends most of their time there, it is not unreasonable to assume that Asmodeus's servant was in fact The Albtraum himself.
== Abilities ==
The horseman is incapable of being permanently killed unless somebody manages to return their head. Other than this, and the powers of the Nightmare they ride, they have no known supernatural abilities, although they was a legendarily powerful warrior even before their centuries of experience in Asmodeus's service. They was also armed with The Chain of the Reaper, until they lost it in Episode 63.
Rygax has stated that when the horseman is killed outside of the Realm of the Albtraim, they are unable to return "for a period of time", as they are trapped in the Realm or elsewhere in The Hells.