Kylo willingly lets himself be the vessel for snoke's next body. Growing inside him is an alien creature that is sapping him of everything, but Kylo is too devoted to Snoke to notice that he's slowly deteriorating. Hux tries to save him but Kylo just rejects his offers of help, thinking Hux is just jealous/wants to see him fail. Eventually Hux cuts the monster out of Kylo and kills it.