Karniel was born at some unknown point in the Neolithic period. He was so old that language was actually a recently-learned skill, so he had difficulty speaking. Physically, he was enormous and strong; almost undefeatable in one-on-one combat.
Karniel managed to do something almost unimaginable: he built an army comprised of both mortals and Immortals alike. Karniel's army carved a swath of destruction across the known world, one day invading the village in which the young pre-Immortal Caspian lived. Caspian was trampled to death, and the village destroyed.
Not long thereafter, Caspian and his first mentor, Bran, confronted Karniel, having managed to build their own army. The ensuing battle between the two forces was later remembered as the Battle of Kamtal. Many mortals and Immortals died that day. Karniel was attacked by Bran, assisted by mortals, ultimately defeating and beheading Bran. He then fought Caspian and his men.
In the end, Caspian managed to defeat and behead him, but Karniel was so old and evil that Caspian received a Dark Quickening.