Death knight lore.

[quote]-Death knights, following full conversion, possess supernatural durability, speed, strength, and agility, being able to punch through reinforced stone walls with ease, dodge volleys of arrows like they were nothing, and tank direct shots from a catapult. Furthermore, thanks to their unique existence, the stores of sorrow an grief they've built up within themselves via ambient leeching can be felt and seen by mortal beings as an aura of blue flame, which often invokes feelings of fear and hopelessness in their hearts. Additionally death knights, like most undead, can survive crossing bodies of water, see in the dark, and sense living beings that might be nearby. Also their mere presence in an area can raise the dead, recent or otherwise.

-Despite their status as immortal killing machines, death knights are far from invincible. They are, like most undead, extremely susceptible to holy or blessed weaponry, unable to cross into consecrated ground, unable to enter areas protected by strong rituals of warding unless said ritual is undone, and, last but not least, easily banished by an experienced practitioner. Of course Osvin managed to circumvent this last tactic via the use of defensive counterspells, but knowledge of the ones he used specifically has been lost to time.[/quote]