The Master above Masters is readying large numbers of warriors as well as his new "artificial" demons. I believe he refers to them as "cyborgs".
He has managed to circumvent the normal path of acquiring "students" in this way, through producing them directly instead of raising them from spawn.
But still... no matter how many he creates a direct assault on any of the other Masters would result in a retaliation. And he simply does not have the energy or resources to take us all on.
It is possible he is planning an attack on the surface, but to what end? He could easily annihilate a walled city but there is no profit there, from what I can see.
I feel like I am always ten steps behind in my understanding, watching from the sidelines as events progress, and only seeing how they fit together after the damage is done.
-log of current events by Demon Master Dzu