The Pledge

The Undead on Pluto were less than three thousand. Most of Pluto's army was destroyed in the previous war and the rest were transferred to the Neptune planetoids.

Varian did a blood bath on a number of Neptune planetoids and created a situation where humans occupied disproportionately more planetoids than abyssals.

The abyssals had to move more forces to the remaining planetoids they had power over or risk getting cleansed out. So, they did and it left Pluto with a very small army.

After the Undead Sovereign woke up a few days ago, he managed to convert ten thousand abyssal corpses from the previous war into Undead. There's no more

death strand to squander. He did it with the whip of death strand in his own body.

But that's his limit. If he wished for the Undead to cross ten thousand, these Undead would have to go to humans and infect them. The infection in the previous war was supported by death strands. Now, it had no support.