Doomsday weapon or Doomsday device come to mind.
There are also "weapons of mass destruction", but that is mainly referring to contemporary weapons, such as nuclear bombs.
Apocalyptic can be used to describe something that can bring about "the apocalypse", where the world changes to an unrecognizable state - usually resulting in the death of most life. A weapon could be described as apocalyptic, but I think Doomsday weapon (or Doomsday device) would suit the phrase "weapon for which there is no defense" the most accurately.
A shorter phrase, and probably the most accurate would be: "weapon that can't be countered" or "weapon that can't be defended against" (which is basically the phrase you already have).