This is Surtor, Teridax's perfect warrior. After he was defeated by Frey, but before he was defeated by Cadowert, he was angry. Very angry. He used lots of his energy to create a warrior that would be able to defeat Frey in combat. But instead of building the warrior from matter, he built him from shadow. Pure shadow. And while energy can be depleted, shadow cannot, for while energy depletes, shadow grows, and so, this warrior does not have even the few weaknesses that the Makuta have. But then there was a problem: he needed to give the warrior a mind, or else he would be a statue, but he didn't have a single corrupted mind which could grasp the concept of pure shadow. And so, he created a clean mind, which is something he never did before, and he fed to it only the knowledge needed to find and kill Frey. But very soon after that Cadowert found Teridax.
Now this creature, who knows only how to kill and wishes for nothing more is after Frey. True, Frey is much more powerful, but even so, nobody can fight against an enemy who can regenerate instantaneously, who can fight relentlessly, and who will never give up, no matter what. It's like betting on the loser - you can't win.