The fan-made game reimagines the Ghosts of Christmas (plus miserable old Marley) as boss showdowns in Mega Man. Sprites, Inc has remade its first game with a new engine, and continues to tweak the difficulty based on community feedback.
The video above should give you a good idea of what it's all about. Future seems to be the toughest Ghost/Master, from the discussion in that forum thread. (The game's five difficulty levels go up to a "Wut" mode, good luck with that.)