Imagining a Death Knight Raid Buff

Death knights were played virtually not at all during the race to world first this season. This is because they do not bring a unique buff to the raid. How would you envision a death knight raid buff? What would it do? Don't worry too much about the numerical balance, I just want to see your idea. Here's an example capitalizing on their survivability:

Grim Countenance Allies deal .6% increased physical and spell damage for each fallen ally.

Feel free to comment on my idea or make your own, or better yet, both!