Eren always knew that there were liable to be repercussions for assimilating Healers' Nirvana into his guild.
He wasn't kidding when he painted a grim picture for Jiana saying there were consequences to taking over her guild. He hadn't said that just to get a favorable deal for himself.
The butcher knew how cultist organizations worked. After all, he was in business with some of them. So he wasn't surprised when he heard that the remaining members of the Midnight Moon along with some others were planning to attack White Raven city itself. It was their way of sending a message.
Eren felt that dealing with cultists was a lot more problematic than dealing with well-established forces or entities. Because he didn't know how to target them. The cultists didn't have fixed assets under their names. So he couldn't hold them accountable for anything.
