Once a month or so, we’ll do a Random Monster Mashup! This could take many forms – maybe see what happens when the monsters fight or team up, think about what kinds of circumstances might result in this situation, and even, if we’re feeling really creative, think about what happens if we stick these two monsters in a teleporter together and hit “SEND.”

In a temple to a prophetic god, a priest awakens, screaming. All she can say, over and over, is this:

Millions walk as one, and their fall shall plague the world!

No one knows what she means, and she can’t explain. All we know is that she insisted on fleeing the country right away, preferable out to sea somewhere.

Now, prophecies in D&D are tricky. Given the capricious nature of the dice and the unpredictability of the Players, it’s almost a certainty that any plans you make as a DM are going to have to be flexible. This is especially true for long-term, campaign-level plans, which is where prophecies usually live. But that’s okay – the solution to that is to wait until you’re good and locked in on whatever terrible thing is about to happen, and then discover that someone had foreseen this eventuality.

Because only someone with divine vision could have seen coming what we’re playing with this month: The Colossus and the Swarm of Rats.

Last month the Randomizer gave us the Empyrean and the Seahorse, and this lives in that same general area. The Colossus is the Dungeon That Walks, incredibly large, very dangerous, and an existential threat to anything in its path. Swarms of Rats are nasty waves of rodents that kind of creep you out and are a little annoying if no one has any good AOE spells loaded.

So how can we put these very different entries from the Monster Manual together into an adventure that pays homage to both of them?

Somewhere on the edges of your world, the Rats woke up.

Perhaps the destruction of an arcane laboratory pumped enough psionic energy into the local environment to create a kind of rodent hive mind. Maybe there’s a planar rift to the Feywild, and its chaotic magics spilled out to uplift the local rat populations to consciousness. This might be an Archdruid of Rats who has finally had enough of humankind and has been casting Awaken twice a day every day until they could build a sustainable colony of intelligent rats.

However it happened, these Rats have started to work together. And because it is in the nature of Swarms to move, we’ll have these rats do the same. They scavenge whatever materials they can find – old armor, bits of furniture, tools stolen from sheds – and start building a walking home.

It starts small, an adorable, rickety, jury-rigged thing, but starts getting more and more complex as more rats join. Soon, rangers and woodsfolk start to speak of a giant rat construct that moves through the wilderness, drawing more and more rats to it, growing larger and more sophisticated as it goes.

Until it can’t be ignored anymore.

Now at this point, you can set this up as a fairly simple adventure – get your Players to this new Rat Colossus, send them inside, battling Swarm after Swarm as they go, until they reach its heart and destroy it.

Ah, but what about the Prophecy, you ask? Good – glad you’re paying attention.

Why, you might ask, would the Rats even build a Colossus? Rats generally live in burrows or warrens. They stay near their food sources and keep a bolt-hole open in case they have to dash away from predators. Intelligent though they may be, this is highly unusual behavior.

Perhaps the effect that Awakened the original Rats had a side effect: Contagion. The force of their Awakened minds is so powerful that it allowed them to uplift more and more rats, but it also drew other things to it. Spirits and psionic entities, malevolent and cruel were drawn to these Founder Rats, who fought them as long as they could until they had to be sealed away. Now there is a small magical seal that binds the Founders together with dozens – maybe hundreds – of these spirits that would destroy them all if they could. And to keep anyone from freeing them, the rest of the Swarm decided that the best thing to do was to keep it on the move.

And so the Rat Colossus was born.

Defeating it means releasing untold number of terrible entities back into the world. If your Players go in there, swords and spells blazing, they could unleash something far more terrible than a hill-sized, ambulatory rat colony. But if they got the Prophecy and heeded its warning, then perhaps they could deal with whatever is at the Colossus’ heart.

They could destroy the Founders alone and release the rest of the Colony from its awakened state, thus destroying a tiny and unique civilization. They could find a way to keep the spirits bound, but separate from the Colony, allowing the Colossus to live on. Will they acknowledge the intelligence of these Rats and the existence of a community that they cannot understand? Will they decide that the lives of thousands, if not millions of Rats is worth extinguishing these malevolent beings at its heart?

Solving the situation is the Players’ problem. Yours is setting it up.

That’s one way to put these Monster Manual entries together. You could try some of these as well:

• The Colossus is literally made of Rats. You can stab it and blow bits up, but it will always re-form. The Players have to learn the secret of unbinding the magic that holds them together if they want to stop this unspeakably large agglomeration of rodents from scraping the land clean behind it.
• Your Players have a classic Colossus – a giant statue animated by magic. But it’s been colonized by Rats. Everywhere they go, Rats. In the joints, behind the walls, in each and every room, Rats upon Rats upon Rats. If your players like fighting wave after wave of creature, this might be for you.
• The Colossus is controlled by a single Swarm. Awakened and adventurous, this Swarm is like the crew of a starship. They sit in the Colossus’ head, piloting it around to seek out new sources of garbage to eat and cats to torment – to boldly go where no Rat has gone before.

However you put them together, remember: prophecies aren’t always clear until it’s too late. ‘Millions walk as one’ could mean a Colossus of rats, or rats piloting a Colossus, or a colony protecting the world from something worse. Your players will have to figure out which prophecy they’re living through – and whether saving the world means breaking it first.