The V'reen morphs were creatures from an alternate dimension released into Elanthia by the elven Archmage Lirou. They were known as morphs for having the ability to shape-shift into those they came in contact with.
The V'reen were released by Lirou in a failed attempt to create an artificial Mandis Crystal in 5103. They originated in the Citadel near the town of River's Rest, but quickly began a migration north towards their preferred colder climates. Though, like a plague, they came to bring misery most upon the Paradis population in the north, primarily Icemule Trace. The V'reen were defeated by several brave groups of adventurers, who discovered the nests and killed the Queens of the V'reen.
The primary details for the event are contained in the The Archmage in the Alchemists