Seath the Scaleless as the first boss kill?
So I have a theory that it's possible to kill Seath as the very first actual boss kill from a fresh start in Dark Souls (Remastered or OG).
Obviously this uses a number of glitches in succession, so bear with me.
1st: Thief + Black firebomb start. Use the firebombs to kill Asylum demon, and exit/reload as soon as his dying animation starts. When you load back in, he'll jump down again as if he never died. You run through the Asylum and talk to Oscar, and he gives you the key to leave. Go open the door and leave without killing Asylum Demon.
2nd: run to Undead Burg, and do the Sen's Fortress gate skip glitch, using the hollow on the bridge to get the overhead killcam. Sit at the Sen's Bonfire.
3rd: The Golem's grab attack launch glitch. If you hit him at the right time, in the right place, during his grab attack animation... it does strange things. One of which if you're oriented correctly is teleporting you into an out of bounds area of Anor Londo. You can't access the normal world from there... so that's the end, right?
4th: nope! Force-quit glitch successfully at this point (if you mess up the minuscule timing for this, you gotta make the Golem glitch work AGAIN, so good luck) and land at the entrance to Anor Londo where the bats drop you off. Voila, you are in Anor Londo with no bosses dead.
5th: There's a golden fogwall in the way! but it isn't the end. If you have fall control, and can meme-roll, you can get an overhead deathcam by rolling off the edge of the lift in Anor Londo, then ride it back up and walk through the gold wall just like you did at Sen's fortress.
6th: Now it's the good ol' mandatory death skip, run through the Archives, run down Crystal Cave, and kill Seath somehow. Probs with the Crystal Halberd.
Of course at this point you are stuck, and the run is over. But I believe this is the furthest into the game you can get without killing anything. All of these have been proven possible individually, but I don't think it's all been done together before, so for now it's just a theory of mine. I may give it a try, but have never gotten the Golem glitch to work the way it should, and still have to land the FQ afterward.
Sharing in case someone wants to try it out, or has any ideas.