Sometimes I hate my players.
My table is currently composed of five diabolically inventive players. They come from a mixed IT/Engineering background, so more often than not, I have to deal with some really weird solutions that they create for the problems I present them.
This is one of those cases.
The group is currently composed of four full spellcasters – two wizards, a cleric, and a druid – plus one gish, a magus. They are currently 17th level, so they have a ton of tools at their disposal.
With that in mind, I created a special challenge for them. It's a really tall tower, in the middle of a lonely, small island on high seas. This tower is populated by a cult of undead-worshippers, with a huge amount of followers and zombies and such. My original idea is that they would get help from the nearby Friendly Kingdom ™, to build a fleet to destroy that evil cult. With that in mind, they played through a introductory adventure, where they blasted the pillars that generated the defenses of the island.
Then, this dialog happened:
"It would take a bit of time to gather the army we need to bash the Dark Ones. I wish we could have a faster way of doing it, without waiting for that slow kid-king to do something."
"If we could just throw a meteor over then, it would be over."
"You know what would be cool? If this was Command & Conquer, we could just aim that Ion Cannon over the tower and it would be over."
Then, my players looked at each other with the most diabolical grin I ever saw.
The result was a few sessions devoted to them building the LOMC – Low Orbit Magic Cannon, a device that mixes up a portal-based particle accelerator with a satellite. It deploys a huge load of magical explosives, from orbit, to any point below.
They did all the math, showed me how to build it, and proposed all the inner workings of the thing with blueprints made on Autocad, and basically made a case why it would be possible to build and what damage it would do. They were really having fun building it, so I let it pass.
They intend to use this contraption to solve the current adventure, and I'm on terms with it already.
However, the presence of the LOMC in my fantasy world will not be ignored. I suppose the news that the existence of this thing will spread like fire, and every single realm will want to control it.
So, here's my question:
How should one DM proceed if his or her group manages to seize control of a superweapon that could change the lives of the entire world? Is it a wise decision to let it roll, or should I take rid of it the sooner I can?
Best Answer
You are about to start a new campaign.
You need to pause the world and have the same conversation, GM and players, as you would at the beginning of any campaign. What kind of setting is this, what themes will you envision tackling, what's your playstyle?
It sounds like you started in a "typical" magic-but-low-tech setting. The setting has now changed. This is no longer your old campaign. Your group needs to decide whether they are interested in playing a new campaign: magic-tech. (Something like the Age of Legends from the Wheel of Time, perhaps?) You need to have that honest conversation with them about whether you want to run that different campaign and whether they want to play in it.
(One possibility, as @SSD correctly points out, is that your PCs are now Misunderstood Villains.)
Or not.
Or the cannon works only once, for reasons. But the real reason in that case is that you didn't want to run a different campaign and you decided not to. In that case you should be honest with your players about what happened.
But notice, you're already having these problems.
You wanted them to play the movers-and-shakers game where they allied with the kingdom, worked the NPCs, and ended up with an army. They explicitly told you that this wasn't interesting and pulled off an (awesome) end run. You and your players are playing different games already. So talk about it.
An historical reflection.
I grew up in a version of D&D where PCs past level 10 were expected to be movers and shakers in the world. Your magic-users were magic-makers, your thieves had entire cities under their control, your fighters were barons and dukes, your clerics had followers spanning continents. They reshaped the world map through magic and through might of arms. The game naturally required transition, in my experience, from one of small-group adventuring to one of collaborative world-building.
Or you started a new campaign with low-level characters and a few familiar high-level NPCs hanging around in the halls of power.
This power progression of the classes--particularly of the spells!--is baked into the DNA of D&D/PF versions. Your PCs--all magic users!--are powerful enough to be affecting the world. And as you say, the world's noticing. It may be time either to pivot or to semi-retire these PCs.