I love Soul Eater!
When Maka and Soul were in Italy, they ran into the Demon Sword Ragnarok, and his Meister Crona. They engaged in battle and Soul used his own body to shield Maka. Crona cut right through Soul and left a mark down his chest. The contact from Ragnarok on Soul's skin infected his body with the black blood. Now, as a result, Soul is visited by the little demon in the black room.
Soul and the Little Demon ^^..