The last arc of the manga Sailor Moon was so brutal, it required the titular character to kill her best friends, the Sailor Guardians. Usagi Tsukino - also known as Sailor Moon - and the Sailor Guardians are iconic heroes who are known for the strength of their friendship. They defend the universe from evil, but the final battle they faced came with an ultimate betrayal and a high cost.
Beginning serialization in 1991, Sailor Moon is one of the best-selling shōjo manga series. Its popularity has brought about three published editions of the manga, two anime adaptations, four animated films, a live-action film, and even a limited-time amusement park attraction in Japan. Sailor Moon is easily recognizable thanks to its team of heroines and their iconic outfits, the sailor fuku - resulting in them being referred to as Sailor Senshi within the manga. They're also known as Pretty Guardians, Pretty Soldiers, Sailor Guardians, and Sailor Scouts - depending on the translation and adaptation being read or watched.
Sailor Moon was created by Naoko Takeuchi. The role of the Sailor Guardians is to protect the Silver Crystal, a powerful artifact. It's sought out by many evil enemies, including Queen Beryl's Dark Kingdom, the Black Moon Clan, the Deathbusters, and the Dead Moon Circus. However, the Sailor Guardians' ultimate foe comes in the final arc of the series, known as the "Star Arc". In this arc, they face the Shadow Galactica, ruled by another Sailor Guardian who seeks power - Sailor Galaxia. This is also one of the darkest arcs in the series, as Galaxia turns Sailor Moon's own Sailor Guardian team against her.
Each Sailor Guardian has a Sailor Crystal. These come from Star Seeds, which boil down to being the soul of a person. When a person is the guardian of a planet, their Star Seed becomes a Sailor Crystal, corresponding to them being a Sailor Guardian. By this point in the series, the bond between Sailor Moon's team has never been stronger, which makes it especially tragic that they are used against her. Galaxia has been collecting Sailor Crystals across the galaxy in her quest to be the most powerful being. Her power only rivals Sailor Moon. Galaxia's conquests have caused her to put many Sailor Guardians across the universe under her control, using bracelets. To obtain the Silver Crystal, Galaxia not only destroys Sailor Moon's friends, but she also resurrects their bodies, making them zombies under her control.
While it's difficult to fight ones' friends, this battle is far more than betrayal. It's the ultimate opposition for Sailor Moon. By switching the loyalty of not just her Sailor Guardian friends, but also her love (Tuxedo Mask), Galaxia hopes to force Sailor Moon into ultimate despair. This would cause her to be overcome by Chaos and destroy her, allowing Galaxia to rule the universe uncontested. However, Sailor Moon isn't so easy to overcome, thanks to her belief in her friends and their broader fight. Though she's nearly defeated by the evil version of her team, Sailor Moon pulls through and destroys their bodies, freeing their Sailor Crystals. The eventual resurrection of the stolen Crystals is supposed to lead to the Guardians' rebirth, which means that by killing the Sailor Guardians' corrupted forms, Sailor Moon has actually saved them. This arc has, thus far, not been accurately adapted in anime, with the first adaptation only corrupting two Sailor Guardians. This arc's brutal for not just Sailor Moon, but also for fans seeing their heroes corrupted. While the series has long since concluded, this is one of the hardest-hitting Sailor Moon arcs and fans aren't likely to forget it anytime soon.