From Sanctuary Shard
There is as little knowledge about the origins of the Jotunn as there is of the elven race, the Alfar they arrive with. Travelers from Earth's Medieval and Hyborian age tell of similar races who worshipped a familiar pantheon of gods, yet the two races were never known to be allied. The Jotunn that arrived with the Alfar were reported to being exceptionally tall, reaching heights of ten and twelve feet with pale bluish-grey skin and a wide range of hairstyles normally styled into locks and braids. However, they were accounted as being generally well featured and did not display the rough features now expected in a Hoggur race.
The Thorn King Era would have a significant impact on the Jotunn, as it was nearly impossible for them to hide among the Alfar and Rohirrum populations. Many fled north and continued to fight the followers of the Thorn King they would eventually turn feral and become the Jotunn we know of today. There are some scholars who have theorized that the Jotunn and the Trolls are of the same bloodline, and while it's possible one must consider the broad disbursement of trolls all over the mainland while noting the Jotunn are almost exclusively southern. It's just more likely that trolls are of another origin and simply earned themselves a proto-nordic name.