A while ago, when Silas left from there, Theron noticed something unusual. There was a servant who was looking at Silas's carriage as he was leaving.
It was very unusual because other than that servant, no other servants were there. That's why Theron was suspicious of him.
He just went and talked with him, asked him what he was doing there instead of working and why he was looking at Silas's carriage.
That servant got scared, and he thought Theron knew the truth. That's why he spilled the tea—that other packs, all the alphas, are thinking of having a grand feast, but that too, of Silas's.
When last time they finally were able to see Silas, whom they had only heard rumors about till now, they were mesmerized seeing him.
But since they know Silas can't become theirs, they are ready to kill him after using him.
