Dropping of the World's Energy
One after the other, the new powerful entities announced themselves to the world.
None of them seems to want to be outdone by the other races, and this moment also provides the perfect time for them to let out all of their powers, and forcing the First Breath to come earlier than anticipated.
Flunra expects the First Breath to come in a couple of days, but that's not the case anymore.
Likewise, Rex lift off the restraint he had on his power and let it all burst forth into one ginormous blast of kingly energy that obliterated the area around him, shocking the rising power with his existence.
It's true that the Supernatural Elders are awakening, but this is not their era.
Rex needs to show them that there are other powers in this era that rivals them, and going about brazenly and arrogantly will be punishable. In case such as this, introduction and first impression is the most important thing.