It is in fact Arthas Menethil's frozen heart. After the fight against Illidan, Arthas cut his heart out and discarded it at Naz'anak: The Forgotten Depths because he believed it was a weakness to him, as he was still human with it.
After recovering the heart from the depths, the Scourge brought it to the Cathedral of Darkness in Icecrown where they were performing a ceremony until Tirion Fordring and his companions infiltrated the cathedral. After the Lich King himself arrived and exposed Tirion and the others, he beckoned the paladin to gaze into the heart.
Tirion looked into the heart and decided that there was no possibility to redeem Arthas, then proceeded to destroy it with the Ashbringer. After the arrival of Darion Mograine and his death knights, the party fled.
Your heart has been grazed by Illidan's glaives. If only you could rid yourself of this useless thing...