The Soul was used as part of a portal to be opened within the depths of the Well of Eternity itself. Before the portal could be opened, however, the Soul's spellwork was disrupted and back in the possession of the resistors. After the Legion was banished from Azeroth and the Sundering of the World finished, Malfurion and the four remaining Aspects placed a spell on the Demon Soul that would prevent Deathwing from ever using it again, and then hid the foul disk in a place where only the four dragons would find it...but not Deathwing.
The Power Unleashed
Ten thousand years later, Deathwing - seeking to rebuild his shattered Dragonflight using the eggs of his archnemesis, Alexstrasza - found the Demon Soul and led the orcs of the Dragonmaw Clan to it.