Kaisel used to be a mount that the Demon King Baran rode in battle. After Sung Jin-Woo's fight, he looks around and found that Iron already killed Kaisellin, dragging his corpse towards Sung Jin-Woo. After he was extracted, his grade was Knight, which concludes he was at least an A-Rank.
Warning!The following is light novel spoilers that have not been covered in the Solo Leveling Manhwa. Click on "Expand" if you wish to read.
He will then die after saving Jin-Woo from the Dragon Breath of the Monarch of Destruction. Even though shadows seem to regenerate infinitely, they cease to exist after being attacked by the Dragon Emperor's Dragon Breath.