[Varyon]

It feels like the most appropriate way to handle such a situation would be, as others have noted, to treat it similarly to the way Berserk works. That said, I wouldn't be opposed to having each use serve as a 1-minute, unlimited use "get out of jail free" card. That is, once it's active, it doesn't matter how much injury you suffer, so long as you stay above -10xHP, when the duration expires, you're still kicking (although any further injury will auto-kill you if you're at or below -5xHP).

As for the build itself, a potential alternative to Limited Use would be Maximum Duration. In that form, once it's active it lasts for some set amount of time (as set by the Disadvantage value), then you have a "cooldown" period during which it cannot be used. One issue you'll run into with just about any similar build is that such abilities need to be activated on your turn, while Unkillable seems like something you'd want to be reflexive - yet Reflexive is often worth more than the discount you're getting. I'm not certain what the best way to address this would be.