Aaron saw right through the Great Horned Rat's scheme.
The Great Horned Rat was probably telling the truth. If Aaron was willing to merge with the Great Horned Rat's divinity, breaking into the Legendary realm should pose little difficulty, because that divinity already encompassed an aspect of Nurgle.
Would some people think there was a difference between comprehending Nurgle's essence himself and merging with the Great Horned Rat's divinity to reach the Legendary realm? That one relied on his own strength, and the other on outside help?
Wrong. In essence, they were the same.
Absorbing the Great Horned Rat's divinity was like receiving a Tantric Initiation. Once initiated, he would understand even what he had never understood before.
And in a certain sense, the Great Horned Rat truly was a deity of "great love." He had always fulfilled his duties toward the Skaven race, considering their interests even as he constantly plotted his schemes.
Aaron remembered that the Great Horned Rat had appeared only twice in the Old World. The first time came after a prolonged Skaven civil war, when he appeared to halt the infighting of the Council of Thirteen and unite the Skaven against their external enemies.
His second appearance came during the End Times. The Great Horned Rat suddenly appeared before the Council of Thirteen and announced that the final hour had arrived. All Skaven were to cease their infighting and prepare for the approaching apocalypse. He also refined a great quantity of warpstone, promising sanctuary to those who proved worthy.
That was where the Great Horned Rat's scheme lay.
The Chaos Gods had been driven away when they bestowed divinity upon Aaron, so the Four Gods could not control him completely. They could only cooperate with him while preparing other plans. But if Aaron absorbed the