The secret of the multiverse? That’s simple, and every cutter in the Harmonium knows what it is: “The Harmonium is always right.” Look, the goal of every enlightened being in the multiverse is to live in perfect harmony with all others. Look around: Peace or war – those are the only true states of the multiverse.
The Harmonium says there’s only one way to have peace: their way. War or peace – squabble among each other or join the Harmonium – those are the only choices. The Harmonium believes that the ultimate goal of the multiverse is universal harmony, and it’s ready to spread that belief to all those other sods out on the planes. If that takes thumping heads to spread the truth, well, the Harmonium’s ready to thump heads.
Sure, there might not be peace right away, but every time the Harmonium gets rid of an enemy, the multiverse is that much closer to the universal harmony it was meant to have.