Conditions for a Rejoining:

1. Tip the elemental balance on a Shard toward a given element.
2. Let the excess elemental aether from that Shard trickle into the Source.
3. After the barriers between the worlds have cracked as a result of this, trigger a catastrophic event on the Source related to that element.

This pulls all of the elemental aether out of the Shard and collapses it on the way, while also magnifying the catastrophe on the Source to unnaturally devastating levels. The reason Emet-Selch mentioned Black Rose has "the perfect aspect" for his plans back in 4.5 is because it's essentially a Light-elemental poison - it halts the circulation of aether in the body, effectively doing the same thing as Light-aspected aether. The unnatural potency of the poison in Crystal Exarch G'raha's timeline is owed to the influx of Light aether from the First, which resulted in the collapse of civilization and the Rejoining of the First.