The people of the Ancient Battlefield called this event 'The Descent'. When it came time for this to occur, those with dreams of returning to the Mortal Plane began to fortify their own strengths. The best way to do that was to subdue the strength of others under your umbrella.
With The Descent approaching, a war wasn't impossible. But for all 18 of their named tribes to lose contact with them… Wouldn't that mean they had already lost?
Communication on the Ancient Battlefield was too difficult. Beasts very much ruled these lands. Their only real method of contact were foot messengers that usually also brought tributes to their tribe every month.