Enligt länken från det lade OP in sitt svar
Title: Barbarian
Author: Ewing, Lynne.
Series: Sons of the dark ; 1
Summary: Having escaped from Nefandus, a parallel universe where he was made an immortal slave, fifteen-year-old Obie must return to retrieve his mother's rune stones in order to fulfill his destiny in modern-day Los Angeles as one of the four Sons of the Dark.