In the following six months, Nial didn't leave the arena once. He sent some of his Infernal Devils and Shadow to follow Milo Necandril to inform all Leaders about his arrival and challenge.
The Leaders were also informed about the fight against the rest of the universe and that they could decide for themselves whether they wanted to fight side-by-side with him, the Asura, the Army of Devils, and the Infernal Ancients.
Nial wouldn't force anyone to fight but it was clear that many would join once he was acknowledged by the Leaders. The four Gigalorian Galaxies hadn't been in a huge war for countless years despite being a race full of warriors that sought the blood of their opponents.
It was only given but they wanted to get back onto the battlefield. Nial's request asked for exactly that.
Most would probably follow his request even without the need of their Leaders' acknowledgment. That was how desperate some were for the blood of powerful enemies.