The Super Mario Bros. franchise is generally very upbeat, positive and cheery in tone, character design and storyline, and the indie RPG Undertale has many things in common with SMB- except for the cheery tone.
Undertale takes place in the Underground, where a human child learns to survive peacefully among monsters who constantly want to murder her, so it's a bit darker than the Mushroom Kingdom.
But Mario is such a murderous madman he fights right in!
Dorkly created this darn fine animated yarn, hopefully you'll dig it!
-Via GameTyrant