BL that i cant remember pls help

okay so i read this BL in highschool and abslutly LOVED it~! and i wanna reread but i cant remeber the name 4 the lyfe of meh.!!!!!!!! HELPPPPPPPPPPPPPPPPPPPPPPPPPPPP

so it was

 servant x master

 lightxdark

the t had a dark sekret that he waz part demon or somethin lik that, and he saved the b when he was a child

thats all i remember plz help meh it waz very popular!!!!!!!