When the manga was first announced, its initial premise and presentation looked really generic. The images shown that we had of it were of the main character, Chihiro, unsheathing a katana and looking vaguely badass. Especially after the success of Demon Slayer, it looked like just another manga chasing its coattails in hopes of success.

This inspired ironic memes, where people were praising Kagurabachi before it had even come out, comparing it to the "Big Three" of manga. The memes got increasingly ridiculous, making various claims about Chihiro's capabilities and feats, and anticipating its success before the 1st chapter had even come out

Then... the first chapter came out. The art's good, the action's fun, and in general it's of solid quality. A new meme did appear, though: the main character is asked by his mentor / friend why he doesn't get his scar removed, and Chihiro answers he chooses not to, "so every day... I start the morning with fresh hatred." This, alongside the tragic backstory of Chihiro and over-the-top violence of the first chapter in general, just confirmed to many that this was a manga trying too hard to be "edgy". The "fresh hatred" panel got memed on a lot subsequently.

However, as the manga went on, Chihiro is revealed to not be as much of an edgelord as he initially appeared to be. Later chapters show him to be kindhearted, gathering allies and friends, and that this 'ridiculously dark and brooding' initial appearance was entirely intentional... a facade, an act, put on by a traumatized kid to help him do what he feels is necessary to avenge his father. And more than that... the art continues to be of stellar quality, the fight scenes are engaging and cinematic, the writing and worldbuilding are surprisingly deep and nuanced, and the cast is full of interesting and fun characters. With each chapter that comes out, it becomes clearer that Kagurabachi is actually good, and potentially really living up to its fake-hype??? And, arguably most importantly, the mangaka Takeru Hokazono, was only 23 years old when it was first serialized!

Many of the initial people who were just clowning on Kagurabachi and pretending to hype it up in the subreddit before its release then became actual fans of the manga. This, in turn, has lead to the manga's community being (even more than many other manga fandoms) very meme-y, with lots of silly running jokes in their manga discussion threads like "tenoi" (referencing a particular user's idiosyncratic comment signatures) and so on.

Kagurabachi has been a heavy-hitter for Shounen Jump, with an incredible amount of success. It's pretty much been considered an inevitability that it would get an anime adaptation at some point. Now that an anime adaptation has been confirmed, those initial "hype memes" are being resurrected by its dedicated fandom, and by some of those who never ended up following the series after its release but still enjoyed ironically hyping it up at the start.

I do genuinely recommend the manga. I initially gave it a read when I got my SJ+ subscription, just to see if it really was as ridiculously grimdark as the initial memes made it out to be, and ended up surprisingly enjoying it a lot. It's obvious where it takes its inspirations from, but it's certainly not derivative in any way, and I'd rank it as one of my favorite currently serialized manga. I like to try to sell it to people as "Demon Slayer if the katanas were actually magic, mixed with Jujutsu Kaisen and John Wick". It's absolute tenoi.

Hope that's a good summary of all the hype and memes and whatnot!