The Mortal Kombat universe refers to the Smash Bros. series’ collection of characters, stages, and properties that hail from the infamously violent line of fighting games developed by NetherRealm Studios.
Mortal Kombat is the only universe to be banned in Japan due to its excessive violence coming into conflict with the CERO Rating system, a fact that Masahiro Sakurai brings up in Mr. Sakurai Presents "Sub-Zero".