00f_by_3va
Anyone know what anime this might be?
The main character is a black-haired guy who attends a school (maybe an academy?) with his best friend, who has blonde or orange hair.
The school trains students to fight or hunt monsters that live in dungeon-like areas.
They meet two sisters at the school—one has black hair (possibly the main girl/love interest), and the other has blonde or yellow hair. I think her name was Stella or something related to stars.
There’s a casual moment where all four of them hang out and get fortune cookies. One of the fortunes might have said something specific about Stella, and the blonde best friend cheered her up—he had a crush on her.
Later, they all go on a mission into a dungeon. Things are going fine, but then Stella gets turned into a monster inside the dungeon.
The best friend is traumatized and ends up in the hospital. The school (which might be corrupt or hiding the truth) wipes his memory or hypnotizes him so he forgets everything.
In the last scene I remember, the main character and the black-haired girl are being attacked—by the school and by the best friend, who now acts like a brainwashed soldier or robot with no memory of what happened.
Vocaverse
I ran searches for anime featuring a school that trains students to fight monsters in dungeon-like areas, involving siblings (especially sisters) and one possibly named “Stella,” but I couldn’t turn up any series that matches all those specific plot points closely.
One partial hit: Chivalry of a Failed Knight (Rakudai Kishi no Cavalry) is set in an academy where students duel. The heroine is named Stella (Stella Vermillion), but here she’s a skilled duelist—not a sister—and there aren’t dungeon missions or memory wipe plotlines  .
It seems the anime you’re remembering might be pretty obscure or perhaps even a one-off OVA or light novel adaptation. The “Stella” name could be misremembered or coincidental, and the unique elements like fortune-cookie scenes, a school cover-up, memory wiping, and a sister turning into a monster inside a dungeon together make it a very distinctive storyline.
To help further, could you share anything else you recall? Even small details could spark a memory—like any visual cues (character outfits, emblem, fantasy elements), where/when you watched it (TV, streaming, dubbed/subbed), approximate era or animation style, or any other character names or specific lines. These clues can be really valuable for pinpointing something obscure.
