What engine is this, and where did it come from? And why is the answer horrifying? (EDIT: a whole bunch of you at once got it - Mickey Thompson Twin Tempest)

The Twin Tempest engine was built by Mickey Thompson from a cut-down Pontiac 195, which itself was cut down by Pontiac’s engineers from a V8. The end result is, as you can see, a slant-two with a V8 bellhousing pattern...

Mickey also ran a supercharged Tempest 4, as well as two other Tempest-powered cars at the same series. See here for more (photos from there).

Hemmings has an article on the engine as it is today.