Khaalthas was away studying at the Kirin-Tor when the Scourge attacked Quel'thelas and never really forgave himself for not being there to assist in the defense of his loved ones. To this day he holds a deep hatred for the undead born of guilt. Upon returning home he found his beloved city in ruins and his family all dead or in thrall to the Lich King. It was Arimylthanis and himself that where tasked with hunting down and destroying those of his family that where unable to find the final peace of death... and as he watch the bodies of his loved ones burn he made a vow to himself, the Lich King and his wretched Scourge would pay no matter the price.