Source of Confidence
A sense of craze can be seen in both Rex's eyes, taunting the Elders sitting in the room.
Stronger people no matter who they are has an innate pride that is associated with them naturally, these Elders surely have that pride inside of them, and would surely not accept Rex's stance of taking the Executor himself.
It was more than a proposition, the fact that he declared it is a mock to the Elders.
"Maybe you need to tone down your arrogance, Royal Black Prince. Don't associate your view on the likes of Supernaturals that you know with us, this is your first and last warning" Elder Nolacula said with a clenched jaw.
None of them know the full story of Rex's fight against the Supernatural races.
But seeing that Rex brazenly walked inside with big confidence and talk big in front of the Elders shows that the newer generations, the likes of Baralt, Solomon, and Saruth didn't do well against him.