I saw a lot of people wanting Samurai as a counter-based tank. Got to thinking, what if they keep in the using your own HP as a oh shit button? Let's say similar to Holmgang (minus the dreaded self bind), but you get a short pacification to take as much damage as possible, without chance of death. Once the ability ends, the Dark Knight unleashes all that built up damage on the enemy? The ability to use your own HP to attack could also link into draining it back, possibly building up a pool for defense or increased healing effects?

Would could be seeing a type of tank that doesn't so much rely on self-healing, and more absolute destruction. Using the darkness inside of themselves, it doesn't matter what happens to themselves, so long as their enemies die.