I had figured it would follow one of two paths:
Its tank stance, in addition to increasing enmity and some base stat, will increase your defense/damage by, say, 10% for every 10% HP you go below 60% of your max. You know, the lower your HP goes, the higher your stats go, and the buff would probably last for a few seconds after being healed up, making DRK more of a steep-hill kind of tank that gets harder to kill as time goes on.

Another way: Your HP dropping by X% below Y% of your max HP gets you a stack of a buff, similar to WAR's wrath, except I imagine it providing a defense buff. So say for every 10% below 50%, 50%-40% gets you one stack, 40-30 gets you another, you get healed up and you keep those stacks, and then you drop to 40% again, that's three stacks. Then you can hold onto those stacks or spend them on buffs, with the number of stacks you have increasing the buff, like increases damage by 5% for every stack you spend, or gives you 10% HP absorb for every stack.