Emrakul is the largest and most fearsome of the three Eldrazi titans imprisoned on Zendikar. She causes silent horror wherever she soars, embodying desolation, emotional and physical distance, the chill of the void, and the terror of being alone. Thus, in many ways, she is perfectly at home on Innistrad.
Emrakul's initial approach to Innistrad pressed on the minds of that plane's inhabitants, straining their sanity. Her full manifestation then amplified that effect. Cultists who bordered on mad obsession descended into gibbering adulation of their Eldrazi idol, and scholars who pried into dark secrets fell catatonic with the full realization of the alien horror they studied.
Emrakul arrived on Innistrad without her brood of drones and other spawn, so she immediately began warping the native creatures of the plane—both natural and unnatural—in her own monstrous image. Beings transformed into Emrakul's spawn manifested fleshy lattice structures, distorted symmetry, and tentacles ending in knobby, vestigial digits. Those who had already begun their descent into corruption before the Eldrazi's arrival were warped almost beyond recognition, but even those who fought against the madness during her approach were susceptible to the life-warping affects of her presence.
Emrakul's arrival set a new series of horrific events in motion, as humans and monsters alike found their very existence threatened by this blasphemous horror.
Emrakul's Madness
The story of Shadows over Innistrad and Eldritch Moon is shaped by Emrakul's arrival and the madness that spreads across the plane around the Eldrazi titan. A D&D campaign set on Innistrad can ignore this aspect of the story and focus instead on the themes of more traditional Gothic horror. But if you do want to add elements of Emrakul's madness to your campaign, use the Sanity score rules in chapter 9 of the Dungeon Master's Guide. Allow characters to make Sanity checks to interpret the effects of Emrakul's corruption, and Sanity saving throws to resist the effects of the spreading madness.
Mental corruption is not the only effect of Emrakul's presence on Innistrad, however. Particularly after the Eldrazi titan fully emerges into the plane, the madness she brings is accompanied by physical mutation as well. Rather than unleashing hordes of spawn upon the world as the Eldrazi did on Zendikar, Emrakul causes the living creatures of Innistrad to transform into her spawn, becoming warped and horrible fusions of normal and Eldrazi flesh. This physical transformation first takes hold of humans whose minds have already been touched by the Eldrazi's approach, with mental obsession often manifesting in physical expression. But Emrakul's physical presence affects every living thing nearby.
A creature or character whose Sanity score is below 10 can begin to manifest signs of physical corruption, which progresses in stages.
Distorted (Sanity 8–9)
Even before Emrakul's arrival, some cultists and those who pried too deep into dark secrets begin to change. In this early stage, physical proportions shift, with one or more of the creature's limbs or some part of the face growing larger, almost always asymmetrically. Noticeable skin blemishes can also appear, most often as boils or clumps of cauliflower-like growths (though these do not yet resemble the lattice of Emrakul's flesh). Creature statistics are unchanged.
Deformed (Sanity 6–7)
In the second stage, a creature can develop a dead or bulging eye, huge tumorous growths, curled hands or feet, violent palsy, oozing secretions from the skin or eyes, raw red patches on the skin, and similar deformities. Patches of lattice-like growths often appear on the skin. In some cases, particularly in Nephalia, cultists begin to manifest distinctly fishlike features—goggling eyes, scaled or slimy skin, webbing between the fingers or toes, or gill-like flaps on the face or neck. Creature statistics remain unchanged.
Disfigured (Sanity 4–5)
In the third stage, the lips shrink away from the teeth, which can rot and fall out. Fingers and toes sometimes fuse together or start transforming into tentacles. Bleeding sores erupt on the skin, or bulbous growths can appear under the skin. Cultists in Nephalia often grow fin-like protrusions or pincer-like hands. At this stage, creatures might manifest improved natural armor or weapons, or simple benefits such as the Amphibious trait of a bullywug, the Blind Senses of a grimlock, Chameleon Skin like a troglodyte, the Slippery trait of a kuo-toa, or the Spider Climb trait of an ettercap.
Misshapen (Sanity 2–3)
In the fourth stage, a creature's flesh often rots away or grows out of control, either leaving muscle and bone visible beneath or concealing features beneath masses of fleshy lattice or bulbous growths. Bone structure can alter radically, creating additional limbs or turning existing limbs into bizarrely jointed tentacles. Extra eyes can appear in unexpected places, or the head can transform into something resembling a fish or octopus.
At this stage, a corrupted creature might begin to transform into an entirely different creature. A human could effectively become any of the creatures mentioned above, or some other vaguely humanoid monster such as a kenku, a lizardfolk, a sahuagin, or a search=yuan-ti.
Monstrous (Sanity 1)
The most warped creatures are barely even recognizable. Some become little more than a mass of writhing tentacles, or lose their physical coherence entirely. Two or more creatures can fuse together, joined by fleshy lattice. The "lucky" ones retain a basically humanoid shape, but otherwise appear completely monstrous, with twisted faces, extra limbs, horny growths, jointed tentacle extrusions, and so on. Such a creature might effectively become an Eldrazi-like monster such as an aboleth, a chuul, a cloaker, a gibbering mouther, or a mind flayer.