Over all not a bad idea. Tho I have to kinda agree with TheOwner in the point that chess is not very elemental. That said I can't decide is the Black King is OP(he make a kick butt Black Queen),UP(that 50 hp hit on his death is gonna hurt),cost to much(that 50 hp hit on his death thing again), out of elemental chacter(50 hp feel very Gravity to me).
Here's an idea. As in chess you are the king and the king is you. Why not make The Black Kings stats equal to your stats With both you and he linked.
I hate to borrow a name from MTG but you could call this passive skill "Spirit Link" or "Soul Bond".
I.E. You play him and what ever your HP is so his becomes and as the Black King take damage so do you, status same. If the King is healed you are too if you are healed so is he. Only insta-kill (i.e. frozen/shockwave) does not effect you. Like a Voodoo Doll that works only on the owner.
The Kings attack is the same damage that your weapon deals with the same effects. No active skills like sniper tho as the King already has an active skill to make a Black Queen.
There are some draw backs to this and several good points too.
Main draw back that I can see is poison a sit'll be like a double hit per point.