Pretty sure no tank immunity ability like Hallowed Ground or Holmgang save you from intentional 1-hit kill abilities like KB's meteor or other such boss abilities, that would potentially break fights. If you look at the ability description for that DRK ability as well as the corresponding PLD and WAR abilities, it says "most" damage, this is why.
Those unfamiliar with tanks in this game, don't ever expect abilities like that to save you from intentional insta-kill abilities because they won't.