Jashin (ジャシン, Literally meaning: wicked heart; evil god; evil design) is the god worshiped in Jashinism (ジャシン教, Jashinkyō). The only known follower is the Akatsuki member Hidan: according to Hidan, Jashin expects that all his followers — so named "Jashinists" — bring nothing less than utter destruction and death. Hidan always used to pray 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 after ingesting his opponent's blood. His skin then turns black with white markings representing a skeleton, a symbol of absolute death. He draws Jashin's symbol on the ground in his own blood and stands in the centre. This symbol is an equilateral circumscribed triangle (an upside down triangle with a circle around it). Once his sacrificial ritual takes place, any kind of pain or injury that Hidan takes is also inflicted upon his sacrifice, be it a mere scratch or a pierce in the heart. However, this ritual can be broken if Hidan either steps or is forced out of the circle. This technique is called "Curse Technique: Death Controlling Possessed Blood."
Hidan is the "successful experiment of the Jashin religion's secret techniques," which grants him immortality. He uses this advantage to otherwise fatally injure himself while he is linked to his opponent in order to kill them.