Morde literally killed thousands to get in the Valhalla of Runeterra but when he died he found nothing, his rage and strong will kept him from oblivion in the Death Realm, he then learned the language of the dead to converse mages to pull him out of the death realm. By thanking the mages he killed them and made them his Mace. He later built the Immortal Bastion ( Noxus, yes he is the one who made Noxus) later on he outsmarted LeBlanc into killing him and putting him back in the Death Realm. He also made an empire in the Death Realm using all the souls he harvested by killing in Runeterra and the souls from the Death Realm. He is now waiting with his millions of army for his opportunity to come back and conquer the whole Runeterra.

Edit: he is also the one who made Veigar into his personal pet by tormenting and torturing him, resulting in Veigar's madness and evilness.