The gods did not know that the one they were preparing to fight was not actually Yaven'Gan, but an illusion created by him. They also did not know that time flowed much slower in Nessus than it did on Primordia.
Yaven'Gan saw that the gods had entered Nessus, but within a half hour Xiuthagurbhahwauhwa came back out. Yaven'Gan took his chance to avenge the death of his ally and leapt from his hiding place in Lotan down towards the god of the underworld. He impacted him with the force of a falling star, pushing him down into the desert sand.
"Hello, Xiuthagurbhahwauhwa. It is so nice to see you." He said mockingly while standing on top of him. "I would love to stay and talk, but I have work to do." He took The Mattock of the Underworld from him and slammed it into his chest, and the god of the dead was sent back to his home.
At the sight of the enemy god the others' armies retreated, and the demons were too busy bowing down before their master to stop them.
Yaven'Gan was slightly disappointed, only seven legions remained. It would have to do, and with his help they should be able to take the world on schedule. He estimated that he had around six months before the other gods returned.
And so the entire surface of Primordia was conquered, with minimal losses for both the Nessians and the people of Primordia, Yaven'Gan was not here to kill the mortals, he needed them for his plan. The few legions of demons were empowered by their god, and most enemies chose to surrender before the great god rather than risk death at his hands. The vast caverns underground were left untouched by Yaven'Gan, for he did not know they were there.
When he had taken the entire surface he put The Mark of the Slave on all mortals on the surface of Primordia, a vital part of his plan. He then awaited the others by the portal, they should return within a month, he had the existence of all mortals in his hands, and if the others did not agree to his terms he would erase them from existence.
(Attack Xiuthagurbhahwauhwa: 1PP. Empower army: 2PP. Mark mortals: 4PP)
(My secret plan was to trap you in Nessus while I took Primordia, I slowed down time there and created an illusion of Yaven'Gan to keep you occupied.
I took the liberty of assuming that your armies retreated rather than being destroyed, they suffered no casualties. Also, the gemchildren are not under Yaven'Gan's control, he did not find them.
Inaluct, if you don't think it is okay that I stole The Mattock of the Underworld from you I will return it later.)