Games like Mayhem in Single Valley
Games like Mayhem in Single Valley include Undertale, Portal, INSIDE, South Park™: The Stick of Truth™, and Turnip Boy Commits Tax Evasion. We matched them by the tags players give Mayhem in Single Valley on Steam (Action, 2.5D, Comedy, RPG, and Zombies): closest matches first, and among equally close ones the better-known games.

Undertale
Shared: Story Rich · RPG · Multiple Endings

Portal
Shared: Puzzle Platformer · Comedy · Puzzle

INSIDE
Shared: 2.5D · Puzzle Platformer · Story Rich

South Park™: The Stick of Truth™
Shared: Comedy · RPG · Action

Turnip Boy Commits Tax Evasion
Shared: RPG · Action · Puzzle

Human Fall Flat
Shared: Action · Puzzle · Puzzle Platformer

Portal 2
Shared: Puzzle Platformer · Story Rich · Puzzle

The Witcher 3: Wild Hunt — Remastered
Shared: RPG · Story Rich · Action

OneShot
Shared: Story Rich · Pixel Graphics · Puzzle

Outcore: Desktop Adventure
Shared: Comedy · Story Rich · Action

Yakuza: Like a Dragon
Shared: Action · RPG · Story Rich

DELTARUNE
Shared: Story Rich · RPG · Comedy

Yakuza Kiwami
Shared: Action · RPG · Story Rich

Fever Meme
Shared: Comedy · Action · Multiple Endings

God of War
Shared: Action · Story Rich · RPG

There Is No Game: Wrong Dimension
Shared: Comedy · Puzzle · Story Rich

Black Myth: Wukong
Shared: Action · RPG · Story Rich

West of Loathing
Shared: Comedy · RPG · Story Rich

Yakuza Kiwami 2
Shared: Action · RPG · Story Rich

The Witcher: Enhanced Edition Director's Cut
Shared: RPG · Action · Story Rich

UnEpic
Shared: RPG · Action · Comedy

Dungeon Munchies
Shared: Action · RPG · Pixel Graphics

OMORI
Shared: Story Rich · RPG · Multiple Endings

ANNO: Mutationem
Shared: RPG · 2.5D · Action