1371 Immortal Dragons.

It is easy to call someone to come and eat but it is difficult to tell them to leave halfway into eating. So the Stellos were reserved and suspicious about receiving the help of the Phoenix race. But the Phoenixes swore that they would leave the plane alone. All they wanted in exchange for helping the Stellos was a certain supreme beast known as the Shadow of Despair.

They don't know why a race that is very rare to encounter in the void universe will suddenly show up for a single Supreme Beast but the offer of the Phoenixes sounded reasonable so the Stellos agreed. 

This led to the cooperation of three different races and a traitor of the Supreme Alliance. It was not by chance that they chose to attack the Stellaris plane when the young supreme beasts were planning to attack the divine plane. That decision was not made haphazardly.