Dark Souls variant is kind of cool, but I feel you'd absorb a lot less damage in total for some of the real tank busters of the game.
With the mind that the other impervious skills should probably also get more flavor (buffs/adjustments) I thought it could be a fun twist on the "you're going to die, maybe" take of Dark Knight's skill in that you make a deal with the void / darkness.. empowering you to the effect that you don't die of course but will if you can't expend that energy soon (collapsing from the dark energy held within, aka too much Edge/Emo lol).
So the change is Dark Knight's blood bar changes to void blood, the void blood must be used up within duration or you'll perish. Void blood may be used as mana or blood, and while it takes priority for consumption the other bars will continue where they left off if you didn't die in the process. Timer pauses and void blood drains slowly if no monsters are around.
Although to be honest I imagine Dark Knight's skill could be a smoother journey if it had a visual notification for healers. Or perhaps if Dark Knight could extend time till death by using their siphon strike (wont increase invulnerability time, but can give healers more time to patch the dark knight).
I think its fascinating how much hype HG has. Holmgang has always made me a bit jealous but I was able to laugh because it locked you down. Now they removed that though for ShB lol.
In an environment where your healer isn't blowing their oGCD in weird spots the cost of going to 1 hp isn't really that serious, so Holmgang is pretty awesome - and if Living Dead goes off without a hitch it's also pretty awesome (unfortunately... sometimes it can turn into a train wreck.. so I would take Holmgang or HG just out of reliability even though LD CAN be great).
That isnt to say I think HG is awful, I just think its interesting that the other skills are actually quite good (LD being annoying sometimes ofc).
Hopefully if people get SE to nerf HG they don't forget about these other skills (nerfing and buffing them all together as need be).