I found it! Not the one - Doom Gruard is from All The World's Monsters vol 2 (1e book published in 1977) created by Dave Hardgrave. It was a 1-24 HD monster.
Description: A magically animated suit of dull black plated armor. Its DEX is always equal to that of it most dexterous opponent. The Doom Guard is completely imune to fire, lighting, FEAR, paralsis, confusion, and all charms. Cold does half damage and slowes it down. The Doom Guard can teleport at will. The only way to kill a Doom Guard is to dismember it and will continue to fight a full effectiveness until killed. When successfuly struck by an enemy, the Doom Guard divides itself into two new Doom Guards, each of which has the same number of hit points as their parent Doom Guard had after it was stuck.