Shamans

Shamans (needs better name, perhaps a leader or a group of leaders to sound less vague), who were powerful practitioners of the Dark Arts, had begun to become corrupted by ("insert demonic equivalent here"). The Orcs (needs better name, perhaps a leader or a group of leaders to sound less vague), who had once battled against their own kind (Orc Civil War needs a name, probably with one faction getting wiped out), were united under the Shamans' power and allied with the Shamans (mind control or willingly?). Together, the Orcs and Shamans turned against the other mortals.

Shamans were able to conjure (choose better work, conjure means to summon a spirit or a being, not a spell. cast would be better) a curse that stripped mortals of their magic, leaving many mortals that were reliant on their magic vulnerable. Many Humans and Elves that depended on their magic to stay alive, were killed off (if you're nothing without the magic, then you shouldn't have it). Orcs, on the other hand, were not dependent on their magic to survive, and their brute strength outmatched the abilities of the Humans and Elves. Soon, the Orcs were able to dominate the Old Continent, forcing the other mortals to push further and further back.

Because the Orc Threat did not mean much to the upper-class Humans and Elves, it was up to the lower classes (focus on higher and lower class doesn't make sense, orc threat would be to evident for higher class to ignore. removing until better reason is introduced) to investigate the impending Threat and the sudden disappearance of the Dwarves and High Elves (disappearance happened 200 years ago, weird time to investigate. removing from timeline and story). These lower-class Humans and Elves eventually came to the conclusion that the best thing to do would be to flee.

Merchants and coastal cities, who were eager to start voyages (unimportant to story, eager makes it seem like they just thought it'd be fun, like there wasn't a huge threat from the orc conquest), sent refugees and lower-class citizens in hopes of finding new land.