We had no idea of the long-term consequences when we killed Gaea.
It took a long time for a god to die, wasting away like Pan had, but it happened nonetheless. After Gaea's defeat it was inevitable. We had saved civilization and preserved humanity, but at an immense cost. Our mother Earth was dying and nothing could stop it. We began to call our planet Earth-that-Was even before we left her.
I was an old man when I took to the sea again. My children had children of their own, and my dear Annabeth had gone to Elysium, content in her accomplishments and a full life. I felt the call to sail, so I did. My ship answered my thoughts effortlessly. I often had ideas for new ships and put them down on paper whenever I could. My designs were very successful and custom ships and sailboats became my specialty. One day I felt a strong wind as I came on deck, and I knew Zeus was speaking to me. I was taken to Mount Olympus, where the gods had assembled together.
"Do you remember the reward we offered you when you were sixteen?" asked the king of the gods. I nodded reluctantly.
"Humanity is leaving, the children of Hephaestus have built mighty ships, but when mortals leave this planet and go into the black they will leave our domain. We need a new god, a god of boats and ships and spaceships. None of the immortals are suitable. Percy Jackson, we require your service. You refused this call once, but we have not forgotten that you have earned it. To be a god is to serve the needs of mortals, and you cannot refuse it again. The power of the gods is directly proportional to the need for them. It is not a reward that we offer you now, it is a duty. Will you take this task, to shepherd humanity on its long journey to the stars, to oversee the ships, your own domain, and to be their home until they reach their new one?"
I bowed my head, thinking of Annabeth in Elysium, then thinking of my children and grandchildren who would soon board those ships and travel to the stars.
"I will," I said, and saw the pride on my father's face. Then I began to change.
Becoming a god is not something I would recommend doing on a full stomach. Fortunately I had not yet had breakfast, but the feeling of growth was extremely disorienting. I was growing in every way. My body expanded, filling up with light and power. At the same time my mind was growing.
I found that I could carry several trains of thought at once. I was looking at my father and wondering how our relationship would change while thinking how I could possibly explain this to Annabeth. Then I was feeling the connection I'd had to ships in the past, but a thousand times stronger. Part of me was a ship, actually, and I perceived that all gods felt this way. My father was his ocean, and Hades was his underworld. Athena was the wisdom that all people had to gain by study and experience, so she was sort of made of brains. Hermes was messages and motion and theft. Hera was marriage and light. Zeus was air and weather. Aphrodite was love and hormones. Apollo was sound and poetry and healing. I was ships.
Every dinghy and sailboat and cruise liner and warship, every aircraft carrier, submarine, oil tanker, life raft, yacht, whaler, and tugboat was now part of my body. I could sense all of them, and since I could be in many places at once, I was there with many of them.
As the god of ships, I spent most of my time aboard them. I was still very fond of oceans, and whenever I come to a planet with enough water to have one, I spend some time in it. I still had the powers of a son of Poseidon.
Many of the gods were unable to leave Earth. Hades belonged to his underworld, and had no power in the stars, though Thanatos was able to collect souls anywhere as well as always, and there were doors to the underworld in many places. Poseidon belonged in the ocean of Earth as well, and when the seas eventually began to boil and evaporate he dissipated into clouds and vapor. His power was broken, but not forever. His spirit still lived through me and the vial of seawater I took from every ocean and sea on Earth-that-Was and poured into the seas of other planets. Part of him remains, scattered across the 'verse.
Zeus lived on, though he was forever changed by the burning of Earth's atmosphere. Rather than the capricious, changeable god of weather he merged with Chaos and became the emotionless, implacable god of the black. Other gods had no trouble adapting. Aphrodite was strong as long as people loved each other, and her daughters founded their own Guild of companions, something of a counterpoint to Artemis' hunters. Ares had less to do for a time as humanity united to leave Earth, but war will always be part of humanity, and Ares is now as strong as he ever was.
As for Apollo and Artemis, they were divided. Apollo literally divided himself from Helios the sun god, who continued to live in the sun, but kept the rest of his aspects and fathered or helped to form a new sun god for each star and protostar when humanity reached its new home. The new, complex solar system had so many planets and moons that Artemis also found a home on some of them.
Each planet and star already had its own god of a sort, we discovered. Most of them were very alien to us, but they grew and changed when people terraformed their worlds and developed their own personalities that reflected the people who lived there. Some of them merged with old Greek gods who had managed to survive. Persephone actually is the former goddess of springtime, though becoming a planet changed her drastically. Spring on Persephone is always a thing of beauty, though.
Hermes would continue as long as there were messages and thieves. Hephaestus was stronger than ever.
Serenity was part of me, but she had a spirit of her own, tasked with calling together the most powerful demigods of her time, uniting them in their vital quest.
My son was the first. He and the daughter of Ares had fought together in the war and sought to form a new crew together. Mal's instant love for Serenity was echoed by her love for him. She was his sister, after all.
Next Serenity drew in a son of Hermes and a daughter of Hephaestus, both very needed for their particular skills. The son of Janus was one she had not intended to gather, but once he was part of the crew she decided to let him stay. The daughter of Aphrodite was very welcome. Finally when the time was right Serenity found the son of Athena and the children of Apollo. Athena's son was the only one of the group who truly understood what he was. Being born without a mortal mother can make one question his origins, and Shepherd Book had looked long and hard to find his. If his faith was more complex and multilayered than other Shepherds, it only added to his wisdom.
Apollo's son was a great healer, but his daughter's gifts put the rest of the crew to shame.
Very few monsters had made it to our new star system. A few still found their way out of Tartarus but many had been completely destroyed when Gaea was. Without the monsters hunting them, many demigods never discovered who they really were or developed their powers beyond simply a strong talent in one field. Still, there was occasionally one demigod whose abilities were so strong that even mortals could identify them. River Tam was one of these.
River had a talent for dance that was exceptional on its own, but her gift as a seer and ability to see into the hearts of men made her a target for the Alliance. She also had an incredibly good marksmanship, which extended to nearly perfect physical coordination in everything she did.