Games like Immortal Mantis
Games like Immortal Mantis include OMORI, No, I'm not a Human, Firework, Amnesia: The Dark Descent, and SOMA. We matched them by the tags players give Immortal Mantis on Steam (Assassins, Psychological Horror, Horror, Gore, and Multiple Endings): closest matches first, and among equally close ones the better-known games.

OMORI
Shared: Psychological Horror · RPG · Multiple Endings

No, I'm not a Human
Shared: Psychological Horror · Horror · Multiple Endings

Firework
Shared: Horror · Psychological Horror · Puzzle

Amnesia: The Dark Descent
Shared: Horror · Psychological Horror · Gore

SOMA
Shared: Psychological Horror · Horror · Atmospheric

DISTRAINT: Deluxe Edition
Shared: Psychological Horror · Horror · Pixel Graphics

Outlast 2
Shared: Psychological Horror · Horror · Gore

The Count Lucanor
Shared: Horror · Gore · Psychological Horror

Resident Evil 7 Biohazard
Shared: Horror · Gore · Psychological Horror

Outlast
Shared: Horror · Psychological Horror · Gore

Ib
Shared: Horror · Psychological Horror · Pixel Graphics

FAITH: The Unholy Trinity
Shared: Horror · Psychological Horror · Pixel Graphics

Tiny Bunny
Shared: Horror · Psychological Horror · Atmospheric

Tales of the Black Forest
Shared: Horror · RPG · Pixel Graphics

Angels of Death
Shared: Horror · Psychological Horror · RPG

The Witch's House MV
Shared: Horror · Psychological Horror · RPG

OneShot
Shared: Pixel Graphics · Atmospheric · Multiple Endings

Mouthwashing
Shared: Psychological Horror · Horror · Gore

Mad Father
Shared: Psychological Horror · Horror · Pixel Graphics

Fran Bow
Shared: Psychological Horror · Horror · Puzzle

Look Outside
Shared: Horror · RPG · Pixel Graphics

Hello Charlotte EP3: Childhood's End
Shared: Psychological Horror · Gore · RPG

Sally Face - Episode One
Shared: Psychological Horror · Horror · Puzzle

KinitoPET
Shared: Psychological Horror · Horror · Multiple Endings