Munchkin Apocalypse is a base set in the Munchkin series with an end of the world theme that includes natural disasters, zombie takeovers, Armageddon, alien invasions, nuclear war, etc. And while this is still a standard Munchkin game where you try to level up by killing the monsters, stealing the treasure, and stabbing your buddy, there's a new 'Seal' mechanic that significantly changes the gameplay.
As well as the brand new 'Seal' mechanic Munchkin Apocalypse also introduces four new classes but does not have races, powers, or other "thingies". It includes Undead and Shark monsters, which follow special rules.
Don't just take our word for it! Check out this review of Munchkin Apocalypse.