Managing a Medieval Low Orbit Ion Cannon

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?