As Gojo worked on breaking through Nyar's reality, the abomination in question was busy dealing with a horror just as undying. Nyar couldn't figure out a way to get rid of Arad, no matter how much he thought or tried.
Even without his full power and mind, AO was still a problem, a huge wall in front of Nyar's plans that he couldn't overcome. Due to Gojo's time magic, Nyar and Arad's speeds had become somewhat relative, with Nyar still retaining the edge. But even then, Arad still dominated due to having four incarnations.
Nyar could've pushed Arad further and destroyed all of his incarnations, but that would be bad for the long term. AO created everything, knew everything, and mastered everything. In fact, Nyar knew what no one else besides Yog knew. AO wasn't even a being, a him or anything, but was one existence of energy, a dark and foggy void boiling with primal power.
