Set was furious with the news Isis had conceived with Osiris. An heir to a throne would threaten his rule. But he knew he would have to focus on one thing at a time. For now, it was keeping the people below him subdued. He also figured he should begin to prepare. While he knew he had an upper hand due to his experience.. that could change in a heartbeat. One misstep and it would be over.
Years had passed, and Isis became a distant memory unless it was Horus related. Set had focused on the expansion towards farther territories in Egypt. However, he had noticed Nephthys's absence. It was irregular at times, but she would return. Then came her disappearance. He had no clue where that goddess could've possibly went to. In the back of his mind, he knew his wife still reserved affection for Osiris. He knew that if Osiris had the chance, he would take her to wife. It made him even more suspicious when reports about a pair of kites were spotted together frequently.
Set had completely detached himself from Nephthys. Not that he suspected anything, he did to a degree. But he had Egypt and was very comfortable as Pharaoh. He could do as he pleased. He could invade another area if he saw fit or burn a town down without quarrel. He was tempted to take a consort or concubine with how often Nephthys left him behind. He was furious when Nephthys had abruptly bore a child. When Nephthys confessed that he was a son from Osiris, he grew enraged. Anubis, the result of the affair, brought shame onto Set in the pantheon. He could not go one step without hearing gossip or hushed whispers that tried to silence themselves as stiff coughs and huffs. With his fury roused against the situation, he threatened extreme measures to silence the gossip. One by one, they were thrown into solitude or worse. Set wasn't going to tolerate it much longer. In the high of the spreading affair, he sent Nephthys to Isis with Anubis along with a letter detailing the affair to Isis. He refused to take her back into his home after that.