Mercury knows the move Aura Sphere, and Herculaneum wanted to use it for a prank and decided to test it out. The problem is that Mercury can't control the move (or at least its power) so the test went awry due to Mercury giving Aura Sphere too much power. As a result the snow wall Herc built not only got reduced to just a hole in the snow, but herc himself got slightly tinged and covered in snow while his brother put up a little sorry face.
