Before, while Sylas was fighting Grin, he had felt that something was off. The Arctic Emperor Cobra had died too easily, and he could feel something particularly sinister coming from the dagger.
However, he had also felt that it was odd that the dagger didn't do the same thing to him when it pierced into his neck. Back then, he had dismissed it, thinking that it was because Grin hadn't used the Skill on him in order to maximize his stealth.
But when Sylas swallowed the Arctic Emperor Cobra's Transmutated Pill, he felt that it was lacking something he couldn't put his finger on. The healing effects weren't nearly as strong as they should have been, as though it was also missing something.
Then he thought about Wills and how both Seeds seemed to require his telekinesis to activate…
Finally, he came to a guess.