An excerpt from Nhoire History:

Little is known about the land of shadows or the creatures that dwell there.  It is rumored that upon entering the land of shadows, your mind slowly starts to meld with the shadows until they become one and you lose who you once were. This theory comes from the study of the fiends that have attacked our world from beyond the rift.

No one knows why but every now and then a one-way rift opens and fiends pour out from the land of shadows.  These fiends seem to be of one mind as if controlled from beyond the rift. They always move towards the capital, attacking anyone along the way.  Attempts have been made to communicate with the fiends but they show no signs of understanding any form of communication, verbal or non. Likewise attempts have been made to capture and study the fiends, however when captured they slowly fade into shadow, disappearing from existence.

There were only two individuals in history that have traveled from Nhoire to the land of shadows and neither has returned. The first being Kaden, lord of the fae, who was banished there starting the Great Fae War.  The other was King Baahl, sent there as punishment for his grievous crimes against the fae. Any future hope of traveling to the land of shadows was lost when the Library of the Arcane was sealed away.

There is a group amongst the scholars that have drawn similarities between the fiends and the fae, notably the ability of the fiends to disappear into shadow as they die.  A similar technique was used by the fae, when they died their essence would disappear and return to Mother Terra who gave birth to them all. This has led to the belief that Lord Kaden may be behind the attacks, which also explains why they go directly for the capital.

The other more logical explanation is that they are predators by nature and are attracted to areas where the population is the largest.  Either way we are no closer now that we were centuries ago in our understanding of the land of shadows.