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Chaos plan was quite simple, he had decided to make the Elder Brain believe he was Jakob and that the Vampires were breaking the treaty of neutrality that the Mindflayer Colonies had with the Vampires. This effectively benefited him greatly, as the Elder Brain ended doing just as he thought he would do, and used his Telepathic Network to communicate what had happened to every important figure within the M i n d f l a y e r C o l o n i e s.
But why would it benefit him if the Mindflayers saw the Vampires as deadly enemies once more? Well, wasn't it obvious? It would make the Mindflayers waste their resources and time fighting the Vampires they met, which would also make the Vampires exhausted from their own resources as they would have to deal with this band of aliens.