This is related to the Mag’har recruitment scenario. You and some Orc (I forget which) fix up some magic to go back to the AU of Warlords of Draenor to recruit some Orcs. There you discover the timeline as ‘caught up’ as it were and thirty or so years have passed. In that time, under the guidance of some Naaru that arrived, Y’rel began converting people to ‘Lightbound’ as the Mag’har call them.
It is unclear what precisely this process does. As well, both sides indicate the other is to blame for some issues with the world related to this war. You help some and escape with a number of Mag’har that join to help the Horde.