Tarkan

Tarkan is the banished spirit of Prince Tarkan, King Veha's adoptive son. He does Sherpa's bidding and is as bound by the Phenomena as Alk and Ilke are if not more.

A mere thought
After being banished, Tarkan spirit became thought/idea, possessing anyone that are weakhearted. The reason King Veha hasn't killed him himself is that doing so will rid the world of people's free will.

Trivia:

 * While the Sha-ra wallpaper was used in marketing for Mørkets Barn was the Tarkan one used on the actual book. Whether this was a mistake or foreshadowing is unkown.