(There are some spoilers here.)

"The Great Fault", Made in Abyss's ninth episode, is well known as anime-original content. Despite the stereotype that usually comes along with that, the episode is widely regarded as solid work that does important things with Riko's character and is simply enjoyable. You could quibble about the ending, where Reg is the one to make the climactic finish instead of Riko, but perhaps this was intended to be part of the point of the episode, to show that Riko could hand a fight to Reg when necessary and wouldn't insist on doing it all herself.

(I'm making an excuse for the show here. It's not flawless.)

It is my opinion that episode 9 is much more than this and that it shows that the show's creative staff fully understood what they were doing. Episode 9 does one very important but inobvious practical thing, which I'm putting in a sidebar at the end, but beyond that it carries a huge metaphorical and mythological charge that is the silent marker and foreshadowing of a phase change in Made in Abyss. This is because of just what Riko and Reg face and defeat at the end of the episode. What they face down and see off is not just any monster of the Abyss; it is the initial monster that Riko and Reg faced, the crimson splitjaw from the very start of their adventure together.

This crimson splitjaw is the gauntlet that has haunted and dogged Riko and Reg from the first episode onward; it was unfinished business from the past. In defeating it, they pass beyond the threshold of the past and enter the unknown, moving on into a new world. The crimson splitjaw was a lingering remnant of their old life that started in Orth, and now they're beyond it.

(Yes, this is a very Campbellian view of things.)

As people who have watched Made in Abyss know, episode 10 will go on to make this change very concrete.

In short, episode 9 primes us for episode 10, not in an obvious way but in a subtle, indirect one. I think it's deliberately designed to do so, since the staff of the anime adoption specifically brought back that crimson splitjaw, not just any monster and not even a generic crimson splitjaw (if they didn't want to design another type of monster). As a result, I like this episode quite a lot.

(This is part of the 12 Days of Anime for 2017.)

Sidebar: The important practical thing episode 9 covers

Episode 9 contains Riko's first serious encounter with the Curse of ascending in the deeps of the Abyss, and illustrates how hard and wrenching it is even in the third layer. This serves as an important lead in to the ascent she goes through in the fourth layer during episode 10, and means that the major impact of the Curse there doesn't come out of more or less nowhere.

(Being told about the Curse in exposition with semi-cute pictures isn't the same thing as having seen it in action the previous episode. The latter primes us much more; it's more visceral and real, especially with how Riko's hallucinations went.)