A teenager in The Bronx yesterday was arrested after stabbing his three younger siblings because he was "seeing demons" and was "zombielike." 17-year-old Nelson Santos was at home in his family's Claremont apartment when he picked up a butcher knife and attacked his two brothers Miguel and James, ages 13 and 6, and his 9-year-old sister Ashley. Santos's mother and a neighbor fetched by one of the brothers eventually wrestled the teenager down and took the knife from him before cops arrested him and took him into psychiatric care. Neighbors told the Daily News that Santos practiced black magic and John Flores, the 19-year-old man who intervened, told reporters, "He said the world was going to end so he had to kill his brothers and sister." Ashley Santos only suffered superficial wounds and 13-year-old Miguel is in stable condition after being stabbed in the chest five times. The youngest brother, James, is in critical condition at Columbia-Pres after being stabbed in the back.