And it's a Mushroom Kingdom gone bad.
Luigi has eaten a zombie mushroom and turned flesh-eater, depicted here trying to eat his brother's face off and wear his guts for garters (zombies having antiquated fashion sense). To defend himself, Mario has a trusty pipe, a knife and a pistol, which for some reason is holstered.
No Mario, no. If there's ammo in that pistol, use the pistol. Inviting a zombie to dance up close is dancing with fate itself.
If you're interested in buying the piece, it can be found (along with a ton of other gaming mash-ups) in KodyKoala's store.