Little is known about these beings, and much of what is "known" is conjecture. The Kal'tesh are believed to be creatures from the stars or other places outside of this world. A group of them, known as the Vrinu, are credited in very ancient times, even before the Alfar, to have had a major hand in creating the world as its residents now know it, to have created the Bloodlines, and various other devices and features. Supposedly, some sort of war between the Vrinu and the other Kal'tesh drove them away from this world long before it was shattered into the Shards.
A few people claim that the Kal'tesh remain out there even today, seeking to re-enter the world and destroy it, and attribute their influence to a red star that was occasionally seen in the sky a couple of years ago. To most people, though, the existence of the Kal'tesh, if they have heard of it at all, is at best a theory.