Before the Convocation, the Death Knights of the Sinistra were the archenemies of the Paladins of the Light. Quite often both parties would forget orders and battle plans, when they met on the battle field. Through friends and enemies with thundering hooves they would crash into one another again and again until one side was completely destroyed.
As elite of the Dark Elves Sorvina has assigned the Death Knights a special place in the hierarchy of the Pact army. Only a fool would trust an ally unconditionally, and that is why Sorvina teaches the Death Knights a spell that helps them to drag the Shadows out of their invisibility, in case they should turn against the Pact.