Elliot knew where Irus got the power to create another realm. There wasn't any other answer to the question.
He only needed to hide the fact that Irus borrowed power from The Manual to create the realm he was supposed to retire to along with Madam Locraine.
"Did he borrow power from the Forsaken?" Frederick asked hesitantly, but Madam Locraine shook her head to say no to this.
"He hated the Forsaken more than anyone I know," the woman said. "And there is no trace of the Forsaken ever since he got trapped in the Abyss. Not even Death can reach out to him."
"Then it's safe to say that Irus is still with us?" Frederick asked, this time Madam Locraine answered him with a firm nod. "Alright. Then we have to wait for him and ask him himself."
Meanwhile, as the Terrans safely arrived in their realm, Irus, Mammon, and Saitan were busy fighting against the synthetic Forsaken that Legion created.