Rise of the Obyriths
In a universe utterly corrupted by evil, thirteen powerful creatures known as the Obyriths focused a tremendous amount of their power into a single Black Shard, then forced it through the barrier separating their world from ours. Then, using the hole created by the shard, they crossed over to our plane.
The Obyriths moved into the newly formed Abyss and attempted to reconnect with their shard. However they were unable to make a move for the shard for reasons lost to time. The Obyriths blended into the demon populace, hiding their identities and biding their time for when they might again retake their shard of unimaginable power.
Every thousand years, the Meeting of 13 Obyriths join somewhere in the Abyss to plan their next moves. In previous millenia, this led to a massive plane-wide war between the Meeting of 13 and the Immortal Council. It is nearing the time for the Meeting of 13 to meet again, and when they do it seems they have numerous plans afoot, each of which would end the world as we know it.