So basically Chapter 19 plays out like it did originally, but you gain him as a green unit and he's actually got some really good stats for him being an umpromoted unit and can be promoted right away since he's about Level 15. Chapter 19 ends like usual but Chapter 20 is a lot different. Chapters 20 and 21 shorten the Parent arc down and the Nohrian siblings get a moment with Garon. Chapter 22 is Chapter 20, and afterwards Anthony explains what happens with Scarlet and is actually more help than he was in the original and is shown more to be a guide than someone who wants you dead. The rest of the game minus 25 and 26 is more world building. The whole Scarlet thing is in 25 and 26 is the real kicker.

Anthony attempts to try and kill all of you at the same time, and then reveals he was Anankos diguised in order to kill the group and Anthony had been long dead like the parents and he goes through a transformation like Berkut did in Echoes. Whatever stats Anthony had before have been capped and he has Luna, Sol and whatever offensive skills he had. Basically if you gave him Lethality, that's kind of a bad idea. After the battle, Anthony is reverted back and he apologizes for doing that, and wishes he could be of more help. With his final breath, he manages to weaken Anankos who's at an insane level of difficulty to a more manageable fight and gives Corrin the Omega Yato.