Before you read the actual story, you should be aware of some things first. Like spoilers regarding both series. Stop larping.
Sunless is NOT the same character as in canon. Not completely, at least.
Think of this as a more extreme version of him with all his worst traits magnified. If canon Sunny is just a normal guy with trust issues, this version of him is practically mentally ill. Although he still seems and acts like Sunny, he will be much more violent, much more distrustful, and much more silly. I've given him some of the Trailblazers personality traits, like their obsession with trashcans. However, they are still seperate characters.
Also, some might be weirded out by the hair and eye color change. It's a volume 1 thing. It goes back to normal in the last few chapters of volume 1.
As for how the two series work, it would be a merger of the two with certain major Shadow Slave characters put into different positions (As of chapter 490, I have only confirmed Nephis' status). This is surprisingly not too complicated, since most of Shadow Slave takes place in the Dream Realm, while HSR takes place on other planets. Basically, imagine Honkai Star Rail, but with the Dream Realm and all related lore. Earth is mostly abandoned by anyone who has the financial ability to go to another planet.
In simple terms, think of this as a timeline where Sunny isn't born when he should be, and the Forgotten One isn't close to awakening yet (If you don't know who I am referring to, expect major spoilers in this fic).
Some major differences I am making would be the timeline of certain Shadow Slave events, and fitting them into the timeline of HSR. There is also hints of Lord of the Mysteries in here, but it won't be explained in depth until much later. (EDIT: The LOTM stuff will be explained in Amphoreus, as everything before that is just hints.)
The events of both series will be occurring, with certain things changed or removed. I've currently only planned until the end of Amphoreus, with certain future arcs planned beyond that though I have not decided how to implement them yet.
This is a harem, but as of chapter 490, the romance has only appeared in small moments, whether literally or in the form of fanservice. There won't be anything real until Penacony.
I'm not really a fan of harems, but I thought it would be pretty fun to try and pull off something that is usually impossible to implement without being badly written. I really could have had Castorice be the sole love interest and make my life easier… but that's how losers think!
Or whatever that one farmer says. But yeah. Don't expect a whole lot of romance besides a doomed pairing or some one-sided crushing until Penacony.
Oh, and I'll be killing off characters. What I wrote above about doomed pairings was totally not foreshadowing.
As for HI3, I haven't played it, and will keep things the same as they were in HSR. No changes to Welt or any HI3 associated lore.
(EDIT: I have become an HI3 larper, and am using information scattered across the internet to implement its lore. If there is anything that doesn't make sense… well, shit lol.)
Both HSR and Shadow Slave are ongoing series currently, so the amount of material I have to work with may limit me. I'll try my best to stretch out the arcs, with at least 100k words for each of them, but if I do catch up(to HSR in particular, since Shadow Slave is much further along), then the story would have to be put on hiatus for awhile. I do not want to have to make new content from scratch lol. I'm simply not creative enough for it.
Assuming that I can continue this story for awhile, you can expect an upload of around 5 or 6 chapters a week, ideally with a thousand words each. I will be spending a lot of time editing the chapters to reach my standards.
Lastly, this is only the first part of a series, with the following ones focusing on different characters, though Sunny will be the overarching protagonist. I've already decided the protags for part 2 and 3, and you can find that on AO3 if you check the series name under this fic on that site.
Furthermore, people have asked me to make smut for whenever I get to that point.
The answer is sure. I guess. Not on WebNovel, though, since the rules are really vague. Over on AO3, I'll have a seperate fic that covers Sunny's freaky escapades for you degenerates. They'll be canon and add characterization, but aren't necessary if you're uncomfortable with that kind of thing.