The Soviets were desperate for a superhuman soldier. They couldn't let on though. Stalin was adamant the courage of the common man would carry the day (or at least the courage of millions of common men). Then Stalingrad happened and in the midst of the deadlock the Nazis brought in the Creature to break the stalemate. He waded through gunfire, leading a platoon of undead Blitztruppen, and slaughtered Red Guards with a sword. He found this Soviet lady sniper they had and broke her neck on film.
Stalin tried to weasel the secrets of the electric troopers and invisibility out of the rest of the Allies but that was a no go. So they fell back on alchemy and hundreds if not thousands of test subjects. One survived. Stalin liked the image of thousands of proletariat sacrificing themselves to create a champion, an amalgam of them all.
Except Mercury was a freaking vampire. Mercury survived his treatment by drawing on the life force of his fellow test subjects. Then the scientists. Then the guards, but he fell to his knees before Uncle Joe and swore to serve him with his dying breath.
They made him a commissar. He didn't take to the front lines often but when he did the results were devastating. He could drain the life from an entire tank crew through the armor. The Russians felt it was better to keep him behind the lines, safe and urging the troops onward. The Nazis had many monsters but what did it matter? The Reds had a worse monster to face if they retreated. They took back Stalingrad from the Nazis.
They're making more.
The commissar is a 7 HD monster. He is AC [AAC] 6  and +7 to hit. He has heightened senses and is only surprised on a 1 in 6 chance.
Explosions, fire, and chemical attacks do only half damage to Mercury. Bullets do half damage.
The creature adds +2 to its initiative rolls and is only surprised in a 1 in 10. In combat his fists and talons do 1d6 damage and he may strike twice per turn. He absorbs half the damage he does and he may use this to heal damage his own damage on a one to one basis.
He can drain life force at a distance through nearly any material (he has de-manned tanks.) This attack has a 20' range and is a cone 15' wide. All those within this cone must make a saving throw or lose 1d3 damage.
More importantly if Commissar Mercury has access to a freshly deceased body he can imbue it with some of his life force to create a revenant. As you can imagine he has no shortage of bodies. This requires six hit points. A revenant has 6 HP, AC 7 and no hit bonus. They can use weapons.
Every six points of damage Mercury does raises his energy level increasing his damage rolls by 1 point to a maximum of 1d6+2. When he reaches maximum damage the next six hit points he absorbs gives him +1 initiative. Mercury can also reduce his own hit points by these amounts to get the same effects (-6, -12 and -18 hits respectively).