Near the beginning of Year 6, the Draught of Living Death is unlocked and becomes available in hub cauldrons. When drunk, it appears to "disguise" your character as a skeleton. (Rather than becoming a skeletal minifig, you become a black minifig with a skeleton painted on it.)
Is there ever a reason to do so? The game has so far offered no explanation as to why I'd need to become a skeleton. Or is it purely cosmetic?