Demons

Some foul souls can't let go of the miserable lives they had. Having been despicable in life, these spirits fester in death. The worst of these hateful dead, through ages of bitterness, gradually transform into Fiends, the tormentors of The Underworld. The people of Theros know these Fiends as Demons.
On Theros, the term “demon” encompasses all Fiends. The people of Theros might refer to “demons” or “devils” interchangeably, having no understanding of the difference between fiendish species or distant nightmarish planes. A Pit Fiend might be interpreted by Theros’s mortals as one of the Underworld’s countless demonic denizens, while a balor meeting a Theros demon on some other plane might consider it a Rarity from some undiscovered Abyssal layer. In general, feel free to use whatever Fiends you wish when telling stories in Theros—the Underworld’s a big place with room for all the incarnate wickedness you desire.

Abhorrent Overlord

Abhorrent Overlords are gaunt, bipedal creatures whose gray, leathery skin is in stark contrast to the gleaming jewelry made of Underworld gold that they adorn themselves with. Their appetite for pain and death is eclipsed only by their greed; these Fiends delight in searching out Treasure and slaughtering all who stand in their way.

Fell Minions.

Harpies, vultures, crows, and other flying scavengers are attracted to an overlord's presence, drawn by its evil and the promise of warm meat. An abhorrent overlord has no sense of loyalty or camaraderie toward these sycophants and often devours Members of its retinue, yet more of the same continue to flock around the demon, offering fealty and squabbling for scraps.

Eater of Hope

An eater of hope is bitter to the core, resentful of all forms of life and joy. Although these Demons can strike down most foes, they prefer to let terror and despair overtake their victims first, letting their victims marinate in fear before The Fiend devours them.

Twister of Wills.

Being obsessed with Wealth and dominating lesser beings, eaters of hope hoard Treasure for the sole Purpose of tempting and manipulating mortals. They revel in using creatures' greed to turn them against one another and their own best interests.

Nightmare Shepherd

A Nightmare shepherd is a gaunt, ashen fiend with leathery wings. It carries a Shepherd's Crook, which it uses to direct a flock of wandering dead that it torments and occasionally feeds upon.

Overseers of The Underworld.

Nightmare shepherds are Nyxborn, drawn from the god-energy of Erebos to be guides of the dead, shepherding them to one of the five wards of The Underworld where they will spend their dismal eternity. They prey upon souls who wander from their rightful places in The Underworld.