In my pinion, the 2 combos conflict with one another. On the one hand, you have the combo that produces all resources and the other is just damage, with no resource gain.

So the first question is, where should the potency lie for the Power Slash combo in relation to the Souleater combo? It isn't just a case of making it stronger, as the Souleater combo provides MP and Blood for Darkside and more importantly, Bloodspiller.

We also run into issues with the Darkside timer itself. Just looking at Souleater and the natural regen, they give enough MP for 1 Edge every 37.5 seconds and 45 seconds respectively, it is these 2 in tandem that allow you to stay MP positive and allow you to use Edge/Flood to keep the timer going, add in a combo that reduces the timer by 15 seconds each time, I guarantee you end up with issues keeping it active. I know you have said allow Syphon Strike to give back more MP, but without knowing how much, it is going to be hard to judge.

Then there is the case of the added effect to Souleater, how much healing is 'a minor amount'. I've said it many many times in the past, but trying to balance HP restoration on a tank is incredibly difficult, it is one of the main complaints that healers have in how their role feels useless. But it almost seems like it was meant to counter the HP cost of the Power Slash combo/Blood Price. So, how much were you envisioning?

Although, talking of HP cost, that is a minefield. It is either going to be a fixed cost, which means it gets relatively cheaper as you gear up, or, it is a % of your HP, which makes it harder to heal you from a lower geared healer (remembering it has to be viable in all content with a variety of healers with a variety of skill/gear levels). We can say the HP absorb from Souleater would counteract that, but then wouldn't that make the HP regen really strong, to the point that healers feel worthless?

So, the 2 combos seem to conflict with each other, rather than complement each other, the MP issue and Darkside timer are concerns and a worry over HP restoration being too strong. It just seems like you wanted to add a combo to the game, but not really thought about how to properly implement it, so you end up with a mess of ideas that don't necessarily work together.