system
Given the following post, respond as if you're commenting on Reddit:
human
[The Matrix] How did the Merovingian keep all of those powerful beings in line?

Why would they work for him, what did he have to offer and/ or threaten them with?
gpt
To paraphrase the man himself, knowing "why" is power. As a veteran of previous Matrix cycles, he was alive for a time when these beings were a known fact, and knows the "why" of how they work. He knows their strengths, their needs, and most importantly if they get out of line, their weaknesses. 

When they first got on his team, the sales pitch was simple; "join or die". The end of a cycle in the Matrix is in many ways an apocalyptic scenario for the programs within, and for any program not slated by the system to be a part of the next cycle, people like him are the only way to stay alive. As these fantasy creatures are 100% not part of the current plans of the Architect and the Oracle, clinging to the Merovingian is the only way to assure any given cycle isn't their last. 

If they get out of line, he has options. They are programs, and programs have weaknesses that can be exploited. For instance, he had a number of henchmen who the Resistance effectively find unkillable... but that Persephone can dispatch in 5 seconds because she knows that they're vunerable to silver bullets. To outsiders they would be an insurmountable obstacle, but to those who know the score, they're just speed bumps.