After the monster took Saki into the closet with it

the larva on the bed starts to wriggling around
it starts to change as he keeps moving it keeps going
until

he is complete

it changed it's shape to look just like Saki and now it's going to take his place in the human world as the really Saki is a prisoner of the monster in the monster realm

(don't worry the real Saki gets saved after his friends find out that this one is fake)