Pretty sure your health HAS to be topped off. It works just like the doom debuff you get in the Wanderer's Palace, where the debuff does not disappear until your current health = your max health. Say the dark knight has 10K maximum hp and the healer is using Cure 2's. If say they use 10K in heals (combination of Cure 2s, Regen, etc) and you take 2K worth of damage during that 10s, then your HP will not be topped off and you will die.