ReBirth The Lunatic Taker

Reiji is your typical high-school student living with his big sister Ayaka. One day he stumbles upon a pair of bracelets on the sidewalk. He picks them up and tries to find the owner but gets sidetracked as a bag snatcher runs his direction. He tries to stop him but is stabbed and killed. On the brink of death he hears a voice from the bracelets "If thou wishes to live form a contract." What has Reiji gotten himself into? And what exactly does the contract entail?