A Decades Old Promise

The Abyssal Emperor's plan was brilliant.

First, the Eight Abysses would release the extra forces and disrupt the status quo of the planetoids.

On Neptune, this extra force—Lightning Wyverns were strong enough to conquer the whole planet.

Second, while the Human army was still recovering from disruption and recuperating, the Undead Army would rise and deal them a critical blow.

Once the Undead army 'woke' up, any dead member on the battlefield would also turn into an Undead in seconds, provided they had a relatively intact corpse.

The Human Army wouldn't be able to destroy all corpses.

What's more, Humans would be outnumbered facing Abyssals and Undead. Even then, in the case that 100 humans killed 100 Abyssals despite being outnumbered, out of every 200 deaths, at least 60 would return as Undead. So, for every 100 soldiers that the human army lost, the Undead would gain 30.