Hello there!
Sorry for not updating in ages. I lost motivation for chapters 8 and 9 so I have decided to make a summary here and continue with chapter 10. I might release chapters 8 and 9 sometime, but I'm not sure yet.
Summary
Chapter 8: once Triton and Percy finish their training, Hermes finds them on the island, where they are with Apollo, because Hermes has a message to Triton from Poseidon and a message to Apollo from Artemis; Poseidon wants Triton and Percy to return to Atlantis and Artemis wants Apollo to tell her where Percy is (she's a little bit stubborn). Hermes also asks Apollo who Percy is and Apollo explains about Poseidon and Kronos and Gaia 'predicaments'. Triton goes back to Atlantis and Apollo brings Percy back to the Underworld, before he tries to make up another excuse to keep Artemis away.
Important note: because Artemis blessed Percy he is able to talk to most animals (like he can with horses, fish, etc.). This might be included in the story later, but I'm not sure yet.
Chapter 9: Hades and Persephone welcome Percy back in the Underworld. Hades asks some of the Fallen heroes (Odysseus, Theseus, Perseus and Atalanta) to teach Percy different fighting techniques with different weapons (Note: I was going to include Achilles too, but I decided I wanted to write that out completely). Percy stays in the Underworld and he starts to see Hades and Persephone as his adoptive parents (Y'know since Poseidon called him a mistake). Percy stays in the Underworld (with occasional trips to his mother and sister and Atlantis) until he turns 10, when he goes to see Sally and Atalanta again with Triton and Hestia. (The next chapter begins after this when Triton and Percy go back to Atlantis).
Another note: I will swap the order of the next two chapters (so next "I hate wearing a crown" then "Camp Jupiter")
I will upload again as soon as possible. I won't work with deadlines since I will only disappoint you all.
So see you soon!
Edit: I have the strange feeling my inbox is going to explode soon...
Edit 2: I am adding/changing some things in other chapters, so don't get too excited yet
Edit 3: yes, I'm alive. I'll be publishing a new chapter soon