Hm, from what I read. I think Rood is already dead, but the demon king's Brother (who will be "Blow") borrowed his body thus reviving him, bringing him back into life for some reason but during the process, the memory of Rood's body is erased, that's why he got amnesia.
If anyone here's ever read Pandora's Heart's manga, must be already noticed that he's in similar situation with the protagonist of this manga, Oz Vessalius.
Well, the reason is, The protagonist didn't know he's the rumoured strongest's 'chains'(this is what they called the demon or monster's here, since this manga have strong reference from Alice in the Wonderland's story) Black Rabbit and the Black Rabbit is using the death body of his own contractor, Jack Vessalius as his host, but ended losing his memories about what happened between him and Jack. The difference is, in his case, it's Jack, who's talking to him inside of his mind when he's collapsed or sleeping (like the way the Demon's king's little brother talking to Rood, when he's in his unsconcious state) so Jack still have control over his own body or rather to be more precise, Jack's controlling the chain who used his body as host.
But in Rood's case, he might be the ones who's being used as host, but he lost all of his memorie during the process, and the demon king's Brother, talking to him sometimes in his sleep or whenever he's fainted, to help him or something else. Anyway, it's still unclear, if the demon's king controlling him or just using Rood's body as Vessel but he lost his own memories (like Oz) or he took Rood's form but losing his memories in the process, then his past self trying to talked to him, to awaken him and make him realizing who's he used to be, like how Ken Kaneki in Tokyo Ghoul trying to talk to Haise Sasaki, even though in Kaneki's case, he's not a dead person from beginning. Or maybe it's Rood (as Rood Sentia) whom made a deal with the demon's king's Brother to bring him back into life for his brother's sake, but he completely lost his memories about it (since there will be a price to pay for a huge deal like that) or maybe it's the Master, himself who made the deal, because it's unclear of what just happened when he's inside in the hell, we only see him crying over the skeleton of Rood (or someone). Sorry if it's really long and it may be pointless, but It's just my opinion of what already had happened.
Read more: http://blackhaze.freeforums.net/thread/61/demon-kings-cause-death#ixzz4TV8BnN5S