Travel to Jupiter was a triumph of the Earth League. It took the most advanced technology, the bravest pilots, and several stepping-stone bases in the Belt to finally reach the distant, waiting giant…and, once there, Earth found the greatest enemy it would ever face – the mysterious and deadly Overlord of Jupiter’s Moons!

It was in 2002 when the first Earth ship reached the gas giant. The crew of the Athena discovered that the four Galilean moons were surprisingly Earth-like, with rich ecosystems and easily breathable atmospheres. While examining the moons from high orbit, they detected a cluster of five odd-looking ships coming in on an intercept course. Prepared to either fight or run (the lessons of the Martian war were still fresh!), they were pleased when the craft stayed out of firing range and began to repeatedly broadcast a radio message. It was a series of mathematical equations, and it began the process of establishing a shared language. After a week of slowly learning the basics, it was clear the alien craft were not hostile and were requesting that the Earth ship land. No Earth radio could reach the mother world from this distance, so it was up to the fleet captain, who decided the chance for peaceful contact with an advanced alien race was worth the risk.

The ship landed at what is now known as the Gray Fortress on Ganymede, home of the Jovian Overlord. There they were treated as honored guests, but all noticed the casual cruelty of their host. Earth history was full of tyrants who smiled at those he could use and slew those he deemed unworthy, and the crew was not taken in by his protestations of friendship, especially when it became apparent he lacked the electron drive and was exceptionally interested in acquiring it. Since he made no outwardly hostile moves, though, basic terms for trade and diplomacy were established.

A year later, in 2003, the Gagarin went missing over Io. Allegedly, the ship experienced some sort of sudden mechanical failure; the Overlord reported that it was seen belching fire and smoke and crashing into the sea. Rescue and emergency teams scoured the jungles and seas of the volcanic archipelago world, but found no sign of the craft, not even wreckage. There was a great deal of suspicion, but no proof, and the Earth League was not willing to risk war across the void without hard evidence.

In 2005, the Overlord attacked. A fleet of electron-drive warships larger and more powerful than any seen before streamed outward from Jupiter. They seized the smaller military outposts in the Belt and executed the soldiers stationed there for “crimes against the Sovereign.” The larger bases on Ceres and Vesta fought back vigorously, taking out a few battleships while sending what information they could to Earth. Then the Overlord’s fleet rushed to the inner system, giving Mars a wide berth and launching attacks on Earth, Luna, and Venus. Earth, caught off-guard despite the valiant efforts of the Belt bases, reeled from the assault.

In the end, logistics, resources, and courage undid the Overlord. He could not adequately maintain and supply his fleet over such distances; he had hoped for a blitzkrieg that would put him firmly in command of the inner system, but Earth proved tougher than he’d imagined. Further, Earth had deeper pockets and a more advanced infrastructure than he’d ever conceived of – his realm knew nothing of “private industry,” so he was quite unprepared when the factories of Earth quickly went from churning out pleasure cruisers and civilian transports to mass producing sleek, advanced warships. Lastly, his troops were motivated by fear, not love of their home world, and were used to dealing with broken and submissive populaces, not fierce patriots fighting for their home world with every aspect of their being.

The willingness of the Earth ships to die for their cause, or of one Earther to give his life for his friends, was something the Overlord simply could not conceive of. Over the months of the war, his fleet was diminished and pushed back. The Belt was reinforced, and the Overlord’s ships were often caught between fleets launching outward from Earth and inward from the Belt. Then the counterattack was launched. It was brief and bloody. The ever-paranoid Overlord had kept most of his native fleet home and not upgraded them to the electron drive. The Earth League, as they had with Mars, constrained itself to limiting the damage to civilian targets as much as possible. The result was a conditional surrender on the part of the Overlord, one that a war-weary Earth accepted. An Earth League base would be established on Europa, and the Overlord would destroy all electron-drive ships and turn over the secret of the gravitic vector, which would allow Earth craft to generate artificial gravity on board.

The situation remains the same to this day. The Overlord’s hatred of Earth is absolute, but so is his fear of what they might do with a second attack. He has been reduced to scheming, plotting, and acting through proxies. Few foes of the Earth League, from the Red Hive to random space pirates, have not received some funding and resources from him – though never in a way that can be proven. The Europan base is considered a hell assignment, a tiny island of Earth League territory surrounded by hostile powers. The Earth League has refrained from actively opposing the Overlord on his own world, despite agitation to do so, at least covertly, as is being done on Mars.

Io

Io is a harsh and beautiful world. About one-fourth of the planet’s surface is taken up by what Earthers have uncharitably named the Scab – a massive region of basalt, obsidian, and twisting rock, strewn with active volcanoes and rivers of lava, forever shadowed under clouds of soot and surrounded by miles and miles of ash-gray seas. The rest of the world is a volcanic archipelago, with thousands of tiny islands, most less than a mile along their largest dimension, and countless semiactive volcanoes. Rich jungle foliage covers the older and larger islands, while many of the newer ones are just bare outcroppings of rock.

The Overlord makes his home in the heart of the Scab, keeping his Iron Fortress safe and stable by unknown means. The vast factories, workshops, and barracks that support his armies and space fleets surround it.

The inhabitants of Io are varied. The Scab is home to a silicate race, the enigmatic and unemotional Rock-men. The archipelagoes hold a humanoid race that maintains a peaceful existence, though they struggle to meet the Overlord’s demands for tribute.

The Rock-men of Io
Io’s native race is a silicon-based life form, one that evolved in the magma pools and then moved out to the land. Slow moving and slow thinking, they gradually developed a culture and civilization. Skilled metallurgists and stonecrafters, they greatly fear the seas and never ventured into the oceans, allowing the humanoid island dwellers to live in peace, until the coming of the Overlord. Now, the Rock-men are the ’; now they only know how to serve the Overlord.

While silicon based, the Rock-men still need to eat, breathe, and otherwise perform the functions of life. Likewise, they have internal organs that can suffer damage. Only in campaigns set primarily on Jupiter’s moons would Rock-men be suitable as PCs; in such cases, they make fine “tanks” but do little else well.

Ionian Lizard-Horse (K’tharg)
This creature is a common riding beast across all of Jupiter’s moons, but it evolved to scamper along the rough and sharp surface of the Scab. It is a sinuous creature, something like a thick-bodied, legged snake, with a long, whiplike tail used for balance. Riding it requires skill and coordination, as it is hideously fast and snaps its flexible body around rapidly in order to turn – or throw an unwary rider. It uses its tail to sweep away predators.

During the 2020s, several were imported to Earth, where they have become something of a status symbol for the idle rich and have replaced ponies in the dreams of a certain class of young girls.

Callisto

Callisto is a cold and harsh world, a world of snow-covered mountains, pine forests, ice-choked rivers and seas, and cold, harsh people. The bulk of the warriors of the Overlord come from this savage world, as the Callistans are people who love war for the sake of war – and once for the sake of honor and glory. The Space Marines of the Patrol often find kindred spirits in these rough-and-tumble fighters, and the two forces learned to respect each other during the Solar War. Legends abound about a second race that dwells on the world, a race of savage, shaggy, man-eating monsters that live deep in the frozen caverns and emerge only when the nights are coldest and blackest, but no one believes such tales.

Europa

“By the infinite seas of Europa!” has become a bit of slang in the System, and with good reason: This pristine globe is mostly water. Unlike the shallow, island-dotted oceans of Io or the swampy muck of Venus, Europa’s oceans are deep and barely cracked by land. Less than 5% of the surface area is covered by ground, and the bulk of that is the island of Tovlor, which has the distinction of holding precisely four square miles of Earth League territory. Various humans, who have become masters of working with local materials, inhabit the other sparse islands of Europa. Aside from a small number of real cities on Tovlor, which benefit from the Overlord’s technological devices, the bulk of the population lives in TL1 or TL2 villages.

The most common form of housing is the quaal shell. A quaal is a shelled mollusk that grows to phenomenal size in the low gravity of Europa. When they die, their air-filled shells bob to the surface, where the locals harvest them. A typical quaal shell can house six to eight adults in cramped, but tolerable, conditions.

In addition to the humans, a race of water-breathing humanoids dwells on Europa; they offer fealty and subservience to the Overlord, but they cannot serve in his fleets or armies with ease, so they are mostly left alone. During the Solar War, the Earth Patrol foiled a scheme to smuggle battalions of mer-folk to Earth, where they would enter the oceans and attack coastal cities from there.

The European seas go down to depths of 10 miles or more, and no one has explored the dark reaches of the hadean depths, not even the Overlord’s men. Rumors and legends of mile-long monsters remain to be verified or debunked.

Ganymede

Ganymede is a strange world, a world where the cracked and pitted surface is filled with lush life. Long ago, the surface of Ganymede split and broke, forming a network of canyons and chasms. In these valleys and depths, rich life evolved, filling the cracks with green. The inhabitants are an agile, furred, and clever species that serves the Overlord as technicians and engineers of superlative skill. The surface regions of Ganymede are covered with mining pits and factories.