unholy aura = 15% movement speed buff for party, removed in patch 3.1. A talent in the unholy tree.

Every DK tank, when tanking, will be in frost presence. And your weapon should have Rune of the Stoneskin Gargoyle. Always. Without frost presence DPS will pull aggro pretty much every fight.

The nice thing about DKs is you can quickly swap gear, change to blood presence, slap on rune of the fallen crusader, and be top 10 DPS. Don't have to change spec, reglyph'ing of course would be nice for more DPS.

Unholy presence is pretty crappy and I don't think anyone ever uses it for anything.