Neptune was the hottest target of all abyssals.
The army was able to hold back the Undead in the beginning. After all, they're just a few thousand and weren't overwhelmingly strong.
But as time went by, more and more Undead began to rise from the corpses.
Of course, the number of Undead rising from the corpses was very less compared to the same during the last war. There were no death strands or death crystals to covert all the dead. So, the efficiency plummeted.
Still, the gradual conversion eroded away the defenses of the planetoid.
The first planetoid fell only after nine days.
The second after six days.
The third after four days.
The fourth took just three days.
Even the conversion couldn't have accelerated the falls to this extent.