That is the Heart of Magic, not actually Malygos’s heart, just a super powerful magical object.
It’s a part of a short questline involving Sapphiron in Naxxramas, the Key to the Focusing Iris and the fight then in the Eye of Eternity to collect the Heart.
Way back in early Wrath of the Lich King it was a pretty good way to get your hands on a nice neck item that was decent item level from what I remember, it’s been a while 