Yeah, chapter 394. That’s when Morena told them to get someone in Tserreidich’s group that they could infect. They want someone who can get close to him.
People assumed it was Bono because he mentioned in an earlier chapter he could transform, so a lot of people speculated it was him using that ability, or Chrollo borrowing it from Bono. There were some who thought it was Illumi as well.