Simple rework to LD: Invulnerability for 10 seconds as soon as the effect activates, but it drains 10% of your HP every second while it's active.
Either have it fully heal you on activation and drain 10% of max HP, or no heal and have the drain snapshot your HP at the moment of activation (so if you use it at low HP, the HP drain is a fixed amount, even if you're fully healed -- risk not using it till you're low, reward is it's easier to heal).
You keep the element of it being a doom timer if you aren't getting healed, as well as the flavor of DRK draining its HP for power -- but as an invuln, its effects are survivable even if you don't get to 100% because your healer burst everything before you hit LD (literally 1 heal and you'd survive), and you can keep your invuln if you were Benediction'd 2 seconds into the effect.
Then it doesn't really matter if DRK gets self-healing effects to bust out of it.
