Who's under the mask and why is he/she wearing it? Well it's MLP Tales' Melody under the mask but it could be any number of reasons why she's wearing it. Perhaps it fell on her head at the Halloween party taking place in the background? In any case, it seems to fit her well but she's probably not fond of the mask's chubby cheeks, which are the whole point of the artwork. Card info:
Mask of Chubbiness
Normal Spell
Remove from play 1 “My Chubbier Pony” monster. Select 1 “My Chubbier Pony” monster you control. The selected monster’s name is treated as the name of the monster you removed with this effect.
Random
Common