If SE really wants each Invuln to be unique, make the DRK's a damage stagger, instead. For 10 seconds, you take 0 damage. At the end of that duration, you get a DoT effect for 50% of the total damage dealt to you, done over 20 ticks (so 1 min; 3s ticks on DoTs, right?). Adjust the % of total damage and number of ticks, as needed.

Plus, this would play out really well with TBN.