People who have already watched Made In Abyss or read the manga know that this story brings out the most horrifying, tragic and horror concepts out of the blue. In manga Volume 3, chapter 19, we have seen the actual dangers of being in the Abyss.
Losing Riko’s arm was one of the most agonizing scenes created by Akihito. While Riko and Reg were trying to ascend to the 4th layer, they had to face the most unfortunate event of Riko’s life. In this ‘Made In Abyss Riko Arm’ article I will let you know why Reg had to cut off Riko’s arm.

What Happened To Riko’s Arm?
After a lot of struggle, when Riko and Reg reached the 4th layer of the Abyss, they came across the Orb Piercer. This monster could throw his poisonous needles to the opponent. And when it started fighting with Riko and Reg, Riko told Reg not to fight with him because his needles can even pierce through metal.
But they needed to fight against him otherwise they couldn’t ascend to the next layer of the Abyss. They tried protecting themselves with the umbrella. But the spines pierced it as well. At some point, the sharp needles almost hit Reg’s leg but missed and their backpack full of instruments was hit instead.
Even though Reg tried to protect Riko from the Orb Piercer, she was hit at the end. Riko remembered the black whistler’s words about the poisonous spines. Soon her hand was swollen and she started bleeding from everywhere.
Reg couldn’t understand how to save Riko. But Riko kept her composure and guided Reg to cut off her arm. She told him that cutting her hand to stop the poison from spreading could be the only solution. Riko suggested that he break the bone of her arm and then it would be easier to cut her arm. At that time, Nanachi entered the scene.
Those who have already watched the series know what happened after that. I only tried to jog your memory before the start of the second season. And those who want to know more about this sad arc- I recommend you to watch the previous season to have the ultimate experience
Conclusion:
Riko’s arm cutting scene showed the beginning of the real terror the Abyss has to offer. And it somehow depicts what the adventurous duo will suffer more later on. It showed the saddest and scariest moment of a friend who had to choose between life and death for a companion.
Now, all we can do now is to wait for made In Abyss Season 2 to know more about the dark side of the Abyss.