Ah! I have a ritual sacrifice thing of similar power-gaining nature going on in my other story, and the answer I came up with: a justicar.

The lord of heresy ("Heresiarch") appointed the justicar to capture and drag criminal warlocks before the collective, basically playing the cop to everyone else's robbers. While the Justicar isn't the most powerful member (that'd be the Heresiarch) nor the eldest or anything like that, the Heresiarch chose this Justicar specifically because he admires and respects honor/good cop behavior; when his motive is predominantly "keeping order", actions against the justicar spur the Heresiarch's wrath, and everyone knows he's 'the cop', it has the other warlocks thinking twice about all the backstabbery and betrayal they've been plotting... at least to work around the justicar's path.

Despite this role/presence, a lot of the story involves the justicar bringing murderous fiends to justice.