Ashmouth

The demonic gateway known as Ashmouth lies in between the Hofsaddel and Needle's Eye passes in the Geier Reach, the mountain range that dominates the province of Stensia on Innistrad.

It is a huge chasm deep enough to glow with magma from below. Ash-ridden smog rises from it, mixing with the dark clouds above. Ashmouth is an infernal gateway, and perhaps the most important one. The demon Shilgengar emerged from this pit, which also spews out bands of devils according to some eldritch pattern only the demons understand.

With Emrakul's arrival, the Ashmouth was utterly transformed. If it once linked to an infernal realm where demons and devils dwelled, that link was altered to pass through some alien region where Eldrazi held sway. The plumes of ash rising from the chasm ceased, and the magma glow in its depths was replaced by shifting patterns of colored lights. Warped and twisted demons emerged from the chasm. Almost unrecognizable, these creatures sprouted extra limbs, additional mouths, great curving horns, and jointed tentacles, all mottled with Emrakul's characteristic lattice of flesh. These horrors started to spread like extensions of Emrakul's will, draining the land of mana and transforming it in Emrakul's likeness.

The Devils were less dramatically changed, but their sense of sadistic mischief became contagious, spreading like Emrakul's madness in their passing.