DIdn't occur to me that'd be a demon inside of Tenzen. I mean thus far we've only seen these ninja rely on their own skills and genetic qualities alone. That's why my first impression of Tenzen reviving himself was that maybe he retreats into his head and re-enters a recessive being inside of him, then gets to work healing his own departed body back to health. Sounds weird I know, but I dunno, it also reminds me of those stories of conjoined twins, where one never fully develops and instead gets lodged like a small disfigured apple in his twin bro's stomach or ribs or whatever like a parasite for life. Or those bizarre cases of cancer lumps that develop an actual set of teeth and hair and stuff.

Anyway, that Obfkufu or whatever talks too slow. I'm also sad Koshiro had to die, cuz he was suffering a lot with Oboro and his never rectified mistrust of the Koga.