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.