It's been explained in earlier chapters in a way but Arc isn't an undead but a dark elf. But because his soul doesn't belong in the world it would have torn itself to bits if his body wasn't "downgraded" into a skeleton. The short time he was in his actual body in the world he experienced all of the suppressed emotions in full in a tidal wave and it made him pass out. Basically the skeleton form mutes his emotions and keeps his mental state calm
Arrrrrrr I thought they would at least show michael in the last page ... the chapter I was expecting the most ...
@Claresilver47 His skeleton form is just a skin that he applied to his dark elf char (so long ago He actually forgot his char was an actual dark elf) it was just applied to the world as is