Shi ga Futari wo Wakatsutomo死がふたりを分かつとも; Shi ga Futari o Wakatsu tomo; Cum parem mors dividit; Even in Death, We're Part of Each Other

Ryuusei saves a demon girl and ends up dying so she shares half of her power with him and now he's basically a demon too. If they separate, he dies. If they don't find the one who summoned her within a year, he dies too.