Since this answer is a riddled with spoilers, read everything beyond this point at your own risk:

I just finished Volume 6 last night, but here’s how it went down after Valiant ran out of raws to translate.

Lugh incites the crowd that came to see his execution in Fomoire then has Nevan Romalung bring in the Alam Karla to denounce the pope and high priests…but the demon managed to gain control over Lugh’s body to force him to kill the Alam Karla. Before he does, Airgetlam, that arm he got back in Volume 5 (the divine treasure he stole from the marquis who tried to frame him for murder) activates and cuts the threads controlling him (Lugh had set it to activate automatically if it doesn’t receive any singal from him), at which point the demon drops the act through his medium, the pope, entirely. Lugh traces the wires running from people in the crowd being controlled to a small house where he uses a new type of bullet made from the demons’ crystallized hearts to kill the Puppeteer demon after a brief fight.

Fun fact: On the way to Fomoire, the academy instructors drug Lugh with sedatives and muscle relaxants to apprehend him without a struggle. He has to piss/shit himself to convince them that the drugs worked. 😐