Darker Than Black II

Ten years ago, an inscrutable and abnormal territory known as Hell's Gate appeared in Tokyo, altering the sky and wreaking havoc on the landscape. The heavenly bodies disappeared, replaced by false stars. During this time, people who possess various special abilities emerged. Kept secret from the masses, these individuals, known as Contractors, are able to murder in cold blood. Various nations and organizations around the world use Contractors as spies and assassins, often resulting in violent battles for information. Each fake star has a corresponding Contractor, and they vibrate when their Contractor's power is being used. As such, Contractors are usually identified by their star's Messier catalogue number. However, their abilities are gained at the cost of their humanity. They are implied to lack human emotions which includes no aversions to murder, and they are regarded as extremely rational and logical in their decision making. Contractors are thus named because of an involuntary compulstion to always "pay a price" each time their power is used (ex. eating flowers, breaking fingers, drinking blood, forcing themselves to vomit). During the series a contractor does mention a rumor of contractors melting if they don't complete the payment, but it could just be a bluff to impress his captive.