@Meddler/RiotScruffy/etc. - Can I get an opinion on an ultimate with a health upkeep cost?

Swain's ultimate increases in cost every second, something that helps keep his power in check and make the ultimate's use interesting (how long do you want to manaburn yourself?). But at the same time, I'm wondering whether or not a similar cost, but in health, could be used for a Champion ultimate.

To be more clear:

Costs X health initially; upkeep cost increases every second. Automatically deactivates if you are below Y% of your maximum health.

Obviously, since this health burn could get you easily killed in long-term fights if you don't turn it off, the benefits of the ultimate would have to be pretty strong.

I'm thinking to adapt something like this onto my Dark Crusader Champion Concept, but I could use an expert opinion on whether or not this would be too dangerous. Note that I don't have an effect for such a skill yet (specifically, what kinds of benefits would it provide), but would like to know whether or not this style of health cost could be too brutal.

Can I get a Rioter's input, please?

For those who are interested:
http://boards.na.leagueoflegends.com/en/c/skin-champion-concepts/s6Yz68uW-nephree-the-dark-crusader