Jashin (ジャシン, Literally meaning: wicked heart; evil design) is the god that the Akatsuki member Hidan worships. Jashin, according to Hidan, expects that all followers of Jashin - so named "Jashinists" - bring nothing less but utter destruction and death. Hidan always prays to Jashin before a fight, asking for a 'good kill.' If he is unsuccessful, or is not allowed to kill, he prays for forgiveness.
When undergoing a sacrifice, Hidan changes appearance. His skin turns black with white markings representing a skeleton, a symbol of absolute death. He draws Jashin's symbol in the ground in his own blood and stands in the center. This symbol is an equilateral circumscribed triangle (an upside down triangle with a circle around it). Once his sacrificial ritual takes place, the victim can be considered dead unless Hidan either steps or is forced out of the circle. Any kind of pain that Hidan inflicts upon himself is inflicted upon his sacrifice, be it a mere scratch or a pierce in the heart. This is called "Curse Technique: Death Controlling Possessed Blood"
Hidan is the "successful experiment of the Jashin religion's secret jutsu", which makes him become immortal. He uses this to advantage to otherwise fatally injure himself while he is linked to his opponent in order to kill them. If he goes without killing people for a certain amount of time, he loses his immortality.