Ryder had planned everything but what he didn't plan was Nyx's such Swift defeat. If he had known about this possibility, he would have been more careful.
He didn't know that Ra could have been even stronger now than he was in the past. He was trapped now. He had killed three High Gods, but other gods were still left.
Moreover, the main opponent was Ra, who was standing between him and his son. And to top it all off, Nyx was out. He couldn't even work with Nyx to engage Ra. Without Nyx, he had nothing. He didn't even have Origin Immunity to survive a conflict.
The only thing he had was the piece of paper that Chaos had given him. It was as if he was drowning, and he only had that small piece of paper to save him.
Despite having that, he wasn't very confident. Still, he wanted to at least try it. If not now, then when?
He had kept that piece of paper with him, mostly forgetting that it even existed.
