With SoV it can be abused too much to be OP, even if it only decreases 2 HP. If it ever chips more, it would be too strong with double draw imo.
I think this is okay with sereprex's idea of having it be a weapon.
Right now, being able to stack them up makes giving it any higher drain level a potential OP problem... As a weapon, you would have to work much hard to do this.
Moreover, depending on how it is implemented, the effect could be potentially blocked by shields.
If that is the case, then I think having the max HP absorb match the damage dealt might not be too bad.
If shields can block the absorb by stoping attack damage, then even the adrena void is manageable.
I think this is a good route to go. It gives darkness a very interesting secondary weapon and helps keep shards interesting.