These formet agents of the Scourge have now allied themselves with the Alliance or Horde. Death knights are a hero class, which means they start at high level. Death knights use runes as their primary resource. Each of the three types of rune is used for different abilities.
Death knights have more ranged capabilities than most melee classes with an emphasis on causing diseases and doing damage with their undead pets. Their primary stat is Strength and also Stamina if tanking.