Games like IMPETUM
Games like IMPETUM include Phasmophobia, Mouthwashing, MiSide, No, I'm not a Human, and Layers of Fear (2016). We matched them by the tags players give IMPETUM on Steam (Psychological Horror, Horror, Supernatural, Hack and Slash, and Walking Simulator): closest matches first, and among equally close ones the better-known games.

Phasmophobia
Shared: Horror · Psychological Horror · Supernatural

Mouthwashing
Shared: Psychological Horror · Horror · Story Rich

MiSide
Shared: Psychological Horror · Horror · Story Rich

No, I'm not a Human
Shared: Psychological Horror · Horror · Story Rich

Layers of Fear (2016)
Shared: Psychological Horror · Horror · Walking Simulator

SOMA
Shared: Psychological Horror · Horror · Story Rich

Still Wakes the Deep
Shared: Psychological Horror · Horror · Walking Simulator

Summer of '58
Shared: Horror · Psychological Horror · Thriller

Amnesia: A Machine For Pigs
Shared: Horror · Walking Simulator · Psychological Horror

Sally Face - Episode One
Shared: Psychological Horror · Horror · Story Rich

What Remains of Edith Finch
Shared: Walking Simulator · Story Rich · Atmospheric

Dollmare
Shared: Horror · Psychological Horror · Simulation

Five Nights at Freddy's
Shared: Horror · Psychological Horror · First-Person

Firewatch
Shared: Walking Simulator · Story Rich · Atmospheric

Poppy Playtime
Shared: Horror · Psychological Horror · First-Person

Outlast
Shared: Horror · Psychological Horror · First-Person

Firework
Shared: Horror · Psychological Horror · Thriller

Outlast 2
Shared: Psychological Horror · Horror · First-Person

DON'T SCREAM
Shared: Psychological Horror · Horror · Walking Simulator

Fears to Fathom - Ironbark Lookout
Shared: Psychological Horror · Horror · Thriller

Visage
Shared: Psychological Horror · Horror · First-Person

Unheard - Voices of Crime
Shared: Thriller · Detective · Story Rich

Blair Witch
Shared: Horror · Psychological Horror · Walking Simulator

Fears to Fathom - Norwood Hitchhike
Shared: Psychological Horror · Horror · Thriller