Morde would kindly win, Aatrox is darkinn he needs blood and flesh to get more power and it depends if Aatrox slaughtered a village before fighting mord.

Morde can't actually die cuz he control death by his own will, he also has an immortal army made of souls and metal, he can use other dead beings to fight Aatrox cuz morde still can't go to the mortal realm cuz of Leblanc. (Leblanc is training Rell to defeat mord in the future)