This completely changes minecraft into an action roguelike adventure. - think Ultrakill. Everything is so polished that you would think that this is a paid game you download somewhere, but it's a minecraft mod. It's very fun to play.

The new golden standard for "Minecraft" Roguelikes

This is a breathtakingly well put together and stable modpack, especially considering it is almost entirely made out of Data Packs, and as the title has said, I really did forget that this is a Minecraft Modpack while playing.

I love every attempt to put rougelike gameplay into Minecraft, and you'll see my reviews on this site back that up, but I genuinely believe this one is special. It is so fully formed in a unique way that others cannot match, not Vault Hunters, not Trepidition, not Roguecraft, not even Minecraft's own 25w14craftmine or the packs to come of it. Even accounting for the deep-pocketed and more mature server dev scene with servers like Telos Realms... Nothing has been able to compare as a total overhaul, this pack truly uses Minecraft as an engine to great effect.

This is not to say it ignores the fact that it is made in Minecraft, it still utilizes the set dressing, visual language, and other shorthands that Minecraft provides well. But without playing it yourself, it's hard to describe just how not-Minecraft this modpack is without that ever feeling like a bad thing. I don't belabor this point to suggest that it should be a stand-alone game, but to commend the team for breaking free of the "uncanny valley" many other attempts at "It's like a game but in Minecraft!" reside in.

Of course, there are a few seams still visible, some menus interact weirdly up close, you have to move around in your hotbar to get to some menu items, there's no voice acting or character speaking noises for dialog, and some menus are navigated with book and quill. But again, unlike the maps, modpacks, and servers you've played before, these don't feel as jarring or restrictive as that all may sound.

This is all to set the stage and preface that throughout the rest of my review, I will be treating this pack as a game, because it succeeds in being the quality of a stand-alone game, and one that would itself earn many played hours from me.

My points of reference

While the game cites Ultrakill as inspiration (and I certainly see it), I'm glad it's not that much like ULTRAKILL.
While I'm a huge fan of 3d shoot 'em up action rougelikes and will never turn down the opportunity to enter a room, bounce off the walls, and shoot hordes of mobs until I get to pick one of three rewards, I often find myself overwhelmed in the most high-octane and fast-paced entries the genre has to offer.
While I've only done about 5 runs (only 2 successfully) I've never felt like I didn't know what was going on or wasn't in control of the game, whether in boss fights or horde rooms.
Even when my screen was so covered in vfx and damage numbers that I couldn't see anything else, or was moving so fast my only ways to navigate were by intentionally running into obstacles I never felt lost.
This game reminds me most of Heart & Slash, Immortal Redneck, Battle Shapers, and in a weird way Deadzone Rogue at least in temperament.

Difficulty and scaling

The difficulty is possibly a little low for some, but it's just right for me.
I don't have a lot of time to play games, and I like that even a successful run is going to be in the range of 30-50 minutes.
I also liked that even my first runs got me pretty far and that there's ample opportunity to get a good and cohesive build early-game, even if you don't get exactly what you want.
Every successful run has me feeling like I'm "broken" and "overpowered" by the time I've made it to the end... of course, all that confidence is dashed away pretty quickly, after the "final" boss fight
That doesn't mean it's without challenge as if you stop pressing your buttons and timing your skills right you will still die quickly, yet not without warning.
Having a relatively limited skill pool and straightforward tech keeps the game from getting overwhelming, even once its cranked up to the extremes, and everything has really good feedback.
While there isn't a tonne of meta-progression in the way I like of permanent skill boosts, there are unlocks that change your per-run progression for the better. These unlock as you unlock higher difficulty levels for your next runs and all feel meaningfully impactful, in addition to unlocking more characters and item unlocks as is typical for this type of game. I've not hit a point where I feel like I just have to grind for a god run, at least not yet.

Builds and replayability.

While there are still some mechanics I don't fully understand or haven't fully unlocked yet, I've enjoyed every character I've tried and could see myself gearing a run towards a focus on any of the three skills for most of the characters.
The characters are all pretty straightforward and understand their role well, and most are surprisingly deep if a little intimidating at first, though thanks to the mostly forgiving difficulty I don't feel like I'm risking wasting my time by trying to learn a new character and have been pleasantly surprised by some of the crazy combos and synergies I was able to accidentally find.
As mentioned above, each run is relatively short yet rewarding even if you don't go all the way. The game does expect you to beat it many times before getting the true ending.
I can easily see myself playing another 30 hours just to scratch the surface of each cool build with each character, and ofc there's a speed run timer if I still wanted to keep playing after that 👀.

The fact I'm even considering that is commendable as in most of the other games I've referenced, and even some of my other all time favorite rougelikes like Rogue Genesia, Into the Necrovale, Megabonk, or Risk of Rain, I really only like playing my favorite character with my favorite build until I perfect it and feel little incentive to try other options (this is why I've never gotten back into even the beloved Balatro).
This game is not like that, and I think that's owed to its relative straightforwardness and simplicity while still offering a variety of content.

Quality of Life & Information availability.

The other contributing factor, however... is probably due to its qol features, good tutorial, and great in-game documentation, stats, and descriptions.
I know I said the rest of this review would ignore the fact that this is a Minecraft modpack, but here I really have to point out how good a job they did at this stuff, not only compared to other Minecraft content but to other full-priced indie and AA games.
Other games need to take note that providing "complex" details to the players who want them is going to be a welcome feature, being able to browse items, and see my stats and equipment in a run is huge.
The hub has just enough room to mess around in without making me waste precious seconds navigating it, not to mention its importance to the story and atmosphere of the game.
The inclusion of a test room with multiple options for dummies is something that really should be a must-have in this genre.

While this may feel like a weird place to talk about this, I also believe how diegetic the game is not only aids the immersive qualities but actually makes it easier to play than if it was a bunch of menus and this is again where I have to talk about Minecraft.
Because this is "vanilla" Minecraft (most of the content is made entirely with datapacks), ATMOS doesn't have the option of fancy menu screens (aside from those literally provided by Fancy Menu, shouts out).
Picking up an item is literally picking up that item from the ground, shopping is also a physical experience, and so is map navigation, cutscenes, menus, almost all of it.
While the devs didn't necessarily have to go this route, I do believe it's born of limitation, like all good ingenuity is, and is better for it. This is one of the main reasons I said earlier that I don't believe this would be better off as a standalone game, because the same qualities that make it better at doing just that than its Minecraft peers also make it a better game period.