A Nightmare is a powerful devil creature like a dire and muscular horse with a burning mane and tail (sometimes with horns too), who come of the 9 hells.

At first as the basics horses, this beast is employed as a mount for some great devilish humanoids on Baator, turning him like a powerful weapon to charge against enemies. But as this creature is rough and with a powerful mind, just some devils can handle and claim him to ride after for the battle and death... So a specific connection is created, or submitted between theses darks souls to "claim" the Nightmare.

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It's the true story in 9 hells but what's happened when the mount and his owner come on a different plan and if the devil's banned without the Nightmare ?

The answer is simple, far from his original plan and without his owner the connection can be broken and the Nightmare return wild and "free".  It's why some mortals can try to handle and claim him too easily when the beast isn?t on Baator. But it's a very hard work and often with the death at the end of running.

This creature who walk in front of you was called certainly for that, but that does not have take place as hoped for it his arrogant summoner...