Concentration for Martials

[Pex]

It looks good on paper, but I think it will fail in practice because generally part of the martial's job is to get hit and get hit a lot. He will fail the concentration check. Not necessarily the first or second time he's hit, but with many monsters having two attacks at minimum concentration will be broken by round 2, 3 if lucky.

If you still like the idea of this virtual resource slot, call it something else that uses a different mechanic. Call it "stance". This allows raging barbarians to have it. You can't stance and concentrate at the same time, and you get a stance check when you are frightened, grappled, restrained, tripped, shoved, or otherwise forced to move against your will. Being paralyzed or incapacitated breaks a stance. You can enter a stance as a bonus action in combat and out of combat just go into it.

What benefit you get from a stance and if you can have choices of stances to be in (only one stance at a time) is left to the reader.