The Nightmares

Attracted to mages like moths to flame, many a nightmare has carried a sorcerer towards adventure or battle, serving him faithfully and nourishing his power.

This sounds almost too good to be true...
And yet, the mages, in their vanity, believe that it isa gift of the universe for them, the only onesfit to ride such noble beasts. Yet still, some avoid black mares, regular and exceptional, like they were pure poison.

Why?

Once, there tread a sorcerer of great power upon the face of the world, and his name was Shawrash, his might and number of followers matched but by his hubris and self-love.
Not enough were garments wovenof sunlight and parades held in his honor, he wanted more,and thus called Arjunoor, the Black Wind of Flaming Winter, from the depths of the Abyss to dohis bidding,and so he spoke: 'Demon,bow before my will, and grant me what no other man has..something worthy of my, my greatness, my character... a jewel beyond competition!'
And Arjunoor smiled, pleased, and replied: 'That what no man besides you deserves shall be yours, a symbol of your greatness!'

And from a patch of midnight, blood of mages and tyrants' fears he made a steed of unsurpassed beauty and swiftness, nobility beyond the imagination of kings and might of a dozen knights, and she was called Shinaiwe.

Shawrash grew even prouder, for who would fail to see his greatnesswhen such a beast carried him, one that tolerated no one else on its back? A tempest untamed and untameable, serving him? And Shinaiwe was pleased, and whispered words of prase into his ear, and made his power grow, as well as his foolishness. She carriedhim into battle, nourished him with her blood,and, in a human's guise, shared his sheets.

Then, the day came when Shawrash no longer could master the forces he unleashed, and his own sorceryconsumed him, and unmade his soul,his very being. The energy set free swept across the lands, and turned vast expanses into realms of chaos and raw magic.

Shinaiwe strode out of the conflagration unscathed, unbound as ever.

...

So, nightmares, the offspring of Shinaiwe, will seek out mages and offer themselves, as but humble minions and servants. Others will simply pretend to be just an exceptional steed.

They willnourish the mage's power until he overloads and bursts,releasing the energy back into the raw stuff of creation, and take some of that force as their own, use a bit of it to create lesser demon servants and store a bit for ... later use. The rest is free to create something new.

The nightmares reproduce through mating - either with any fantasticequine (unicorn, hippogriff, centaur...) or when shapeshifted, with their 'master'. To bear a child, they must have energy from a previously burst mage within, and this is used up. If the childis male, it will share the father's species, if female - a new nightmare is born...

Theirs are the powers of illusion and shapeshifting, as well as of swiftness and speed.
Also, their reputations for being the minions of Hell is well deserved, as their control over fire magic is exceptional.
The are able to communicate with their master through any mirror, his reflection speaking to him.
Other than that,they are resilient and highly agile.

While any powerful mage could tempt a nightmare to do his bidding, very few possess the reason and will to resist its lure and power, and thus suffer a messy death.