The nightmares are most feared among all creatures which stalk the Dreamworld, and rightly so: for they are utterly unnatural. They do not fill an ecological niche, do not procreate and spread life, do not have any ancestry or native origin: they simply do not belong. Intruding from abstruse planes of existence, they impose their aberrant corporeal forms to one end- destruction.

Specifically, the destruction of the dreamkeepers.
Thirsting from beyond time to undo the tapestry of creation, they level their nihilistic vendetta against sentience wherever it may be found - and desire exceptionally to wipe humanity from existence.

Incapable of bringing form to themselves in our reality, physically butchering humankind is a fantasy beyond their grasp. But like a swarm of ravenous vermin, they voraciously scrabbled against the borders of our sphere until they found its weakest threshold... Our unconscious dreams.
Lusting to penetrate that thinnest of veils shielding human consciousness from their malignant influence, only one thing blocks their touch: the spiritual bond between a human and their dreamkeeper. As long as a dreamkeeper lives, their human counterpart is protected within that bond - beyond reach of the nightmares.
As long as their dreamkeeper lives... A condition which the nightmares are only too eager to alter. Capable of unlocking human vulnerability with the premature murder of a dreamkeeper, their ultimate ends are far more ambitious in scope. Absolute eradication of every last dreamkeeper would leave all of humanity exposed, victim to the whims of the nightmares' nonexistant mercy.

Successfully seeding their presence in the Dreamworld aeons ago, they continually manifest there to fulfill this purpose.

They can enter the Dreamworld at only one location - the fabled desolation of Bralgu, where they incubate within the twisted nest of evil, Malangi.
Nightmares do not know childhood - they coalesce within Malangi until they attain their completed physical form.
Though supernatural, their manifestations have certain limits. As individual entities, some nightmares possess more power than others, and that power can be developed in different ways as they gestate.
Some nightmares may opt to express their power in maximum physical form, attaining colossal size and strength. Others may choose to mitigate their physical might, and focus the remaining potency of their being into residual supernatural abilities.
Choice is limited, however, when it comes to appearance. A nightmare will naturally cultivate a form which expresses their intrinsic quintessence - which is why they frequently appear as horrific beasts. Only nightmares with noteworthy willpower can influence their morphology otherwise, and then only by degrees.

An additional limitation on nightmare manifestations is their cumulative threshold. The more nightmare power exists in the Dreamworld, the more difficult it is for further nightmares to manifest. When their collective presence peaks, it is impossible for more nightmares to gestate. It is the innate metaphysical balance of the Dreamworld that causes this phenomenon. Were it not for this naturally limiting threshold, innumerable nightmares would smother reality.

In addition to metaphysical constraints, manifested nightmares are burdened by the task of maintaining a living body. They must feed and care for themselves like any other creature, or face death.

Physical expiration is less significant to a nightmare than it is to other beings, however. Although frustrating and insulting, death for a nightmare is only temporary. Their malignant spirit can instantaneously return to Malangi, and again join the queue for manifestation.
Despite their relentless nature, the nightmares suffered a cataclysmic defeat centuries ago. Diminished and endangered, they concealed their existence for generations, their bitterness simmering in confinement as they slowly, incrementally regenerated their strength. Abiding within the patience afforded by pure malice, the time of their reprisal is drawing near - unsuspected by the dreamkeepers.