I am building a fantasy world roughly the same size, elemental makeup, and atmospheric composition as Earth. In this world, there is an island (about the size of Australia) known by outsiders as the Cloistered Realm for its extremely high rocky walls 100km in height which bar entry from all sides, save for two cavernous passages.
At one point, it was a normal island whose southern coast is about 60° south of the equator. After a mysterious, cataclysmic event ~5000 years before the start of the story, it is now sparsely populated by humans, mineral rich, has oodles of volcanic activity (similar level and type of volcanic activity to Yellowstone Park), and an everlasting night (the sun, mysteriously, never rises within the walls, but the moon does appear normally - the length of time the moon stays in the sky varies on the time of the year).
How might the wildlife adapt to such an area? What might it take for large life forms (dense forests, grassy hills, predators the size of large bears, etc) to survive/evolve?
EDIT: Magic of six varieties (light, shadow, fire, water, wind, and earth) is readily available, even to wild animals, plants, and fungi. It is "fueled" by the same biological energy that these creatures use for their own physical processes and all creatures are limited to one of these six varieties.