Letting his guild join the Cult? This would serve perfectly well towards his goal to find and nourish many masters with Mystic Arts powers in his guild. Not to mention he wouldn't just join the Cult, he'd also form an alliance with them, would join the Cardinal Council as a formal member, enjoying the endless benefits from doing so.
To him, this was just a dream. And he didn't get why the Cardinals' stance changed one hundred and eighty degrees, changing from seeing him as a tool into one worthy to join them.
"What's the catch?" After thinking about this for a few minutes, which the Grand Cardinal and others kept their silence during, William realised there must be something they were desperate for, something they needed him to do.
And from such a generous offer, he expected such a task would be quite dangerous. After all the Grand Cardinal said it, they planned to establish a new division, "you do know I have zero experience or power in terms of Mystic Arts."