After collecting the last Empty Box, return to Sabitsuki's Dream Room and take the new elevator down to the Rust Corridor. Navigate the corridors to their end and defeat the Inner Demon. When you return to Sabitsuki's Room, leave through the door.
Rust will leave her room and head for the roof of her building. Similar to the Cleaner Ending, she will be interrupted as she stares off the edge by a Cleaner. The Cleaner will brandish a chainsaw as she approaches Rust, who eerily smiles in turn, approaching the Cleaner while giggling. Rust will then charge at the Cleaner, who, in self-defense, appears to cut off Rust's legs, leaving her bleeding on the rooftop. The screen fades to black, before fading back into the Cleaner carrying Rust's body, just like the Cleaner Ending, before fading away. The screen will fade back into a scene similar to the other two endings, with two Onigo surrounding Rust, before the screen abruptly cuts out with a laughter.
A cutscene will then play, showing Rust, wearing her headphones, walking across a black screen as various important NPCs seen in her dreams pass by. She walks off screen after passing by a bowing Cleaner as the credits appear.
Spoilers end here.