This is a brief explanation of points of this story that may confuse you:
1: Raenef, from Demon Diary, will also be in this story. Because there is no "multi-crossover" button, I guess I'm just gonna warn you here.
2: Raenef ish gonna be an assassin. He joined with Train and Creed after meeting them in a market.
3: This story will take place before Creed went loony. Saya never came up, and Train and Creed are partners.
4: Raenef is their partner, also. Train is the black cat, Creed is the gray cat, and Raenef is the white cat. They are an assassin group known as the Chronos cats.
5: The homunculi Ling!Greed, Envy, Wrath, and Pride have agreed to work with the military peoples because they're my favorite homunculi dammit stop questioning.
6: For whatever reason, Edward, Alphonse, Winry, Mustang, Riza, and May have all taken up residence in the Armstrongs' estate. The homunculi came with them. Don't question it. It's necessary for the storyline.
If I think of anything else you'll need to know, I'll let you know later.