Fear Aura (Su): Death knights are shrouded in a dreadful aura of death and evil. Creatures of less than 5 HD within 15 feet of a death knight that look at the death knight must succeed on a Will save or be affected as though by a fear spell from a sorcerer of the death knight's level. A creature that successfully saves cannot be affected again by the same death knight's aura for 24 hours.
Do we want to keep the limitation of HD? This would fit with the death knight and the lich, to be sure, but would be little more than flavor for the sorts of PCs who'll end up fighting one.