Quick walkthrough for finding the secret in Chapter 9. Obvious spoilers.
Find the Wow
The Farewell DLC for Celeste added a new brutally difficult chapter, as well as a secret within. Those who viewed the achievement list might've seen an additional hidden achievement added:
Wow - Find the moon berry
You can find it in assist mode, but the fun is in the challenge! Other than it being safe to assume it's located in Chapter 9, there isn't much in the way of clues. This guide details how to find the berry, but before revealing it all, here are a few hints in increasingly revealing order:
- It isn't the golden strawberry (yes, there is one for this chapter, good luck).
- Pay attention to your surroundings.
- It's in probably your most dreaded location.
- You're expected to die at least once.
Here's a more comprehensive version. For starters, you're expected to know about:
- Everything taught to you in the entire game including this chapter.
- How to gain height diagonally using the jellyfish glider. When holding onto the glider, release it, dash diagonally upwards, then grab it again.
- How to gain additional vertical height using the jellyfish glider. This only works when throwing the glider against the wall or a spring, such that it bounces back for you to dash upwards and grab it again. This gives you an additional vertical boost.
Now, the start of the secret is located at the end of the very last room i.e. the longest room in the game. This means you'll have to do the entire room if you want a shot at the secret, though fortunately you can reach it directly from the checkpoint select.
After disabling the electrical fields, dash immediately back left onto a floating pillar, then make your way up. This is fairly long gauntlet to the top, and you will need to do a few wavedashes and wallbounces on the way. Some floating pillars descend from your weight - if you can't make the next jump then jump and dash upwards in place so that the pillar rises again. The area with the puffer fish is one notable chokepoint - you need to be blasted to the right far enough so that you can reach a platform. If you miss it though, you can hold left for most of the way down and you should land back on an earlier part of the gauntlet. Even more mercifully is that unlike the rest of this room the developers were kind enough to add a checkpoint after a section with the jellyfish glider, so if you mess up after that you won't need to do the entire room again.
Once you reach the top, you're greeted with two rooms linked together, and a disconnected passage. There doesn't appear to be any way to reach that passage naturally, but if you make contact with the spikes on the right of the lower room you'll respawn inside that passage. Go through, and you'll be presented with the second half of the gauntlet.
The second half tests your ability to use the jellyfish glider, which is why knowing about points 2 and 3 above is necessary. To start your ascent, grab the glider and throw it against the rail walls, such that it bounces back so you can grab it again. Alternatively, walljump off the rails and do a diagonal glider dash to the next rail. The second part has you doing mostly diagonal glider dashes - you're able to skip a few springs and walls if you're feeling adventurous.
A vertical glider jump will take you to the next section with crumbling platforms. For the spring at the end you're expected to throw the glider onto the later set of springs, then dash up and grab it again. The sweet spot to throw it is approximately where the floating wall is. The section following that consists of more diagonal glider dashes; a straightforward test of skill.
Contrary to what the binoculars at the bottom might suggest, there is a final section above this with two main obstacles. If you want to plan ahead then you're able to stay suspended in one place due to the updraft, otherwise get ready. Firstly, make your way to the left, throw the glider right so it unlocks the key, then dash your way around and grab it on your way up.
The last obstacle is annoyingly precise to setup, and may be the cause of a lot of frustration. Make your way to the left path, try and almost hug the left wall (not too far or the recoil from throwing will kill you), then throw the glider at the very top. If you did it correctly, the glider will hit the set of springs on the right and bounce back. Quickly dash around through the middle path and grab the glider again, preferably with an upward dash. Hold the glider all the way to the end and you're done!
For an easier to follow walkthrough, check out this video. Skip to 2:06 for the start of the gauntlet.
This video also shows the ending after the secret (it's no different from the regular one).