Among which religious group do some practice 'Tatbir' in their religion?

The practice of Tatbir is a form of ritual bloodletting, practiced as an act of mourning by some Shia Muslims (it is a forbidden act according to some Grand Ayatollahs), for the younger grandson of Muhammad, Husayn ibn Ali, who was killed along with his children, companions and near relatives at the Battle of Karbala by the Umayyad Caliph Yazid I.

The practice was first introduced by the Qizilbash tribe who were instrumental in establishing the Safavid rule. Tatbir is a contested issue among Shia. Most clerics deem it to be self-harm and hence 'haram' (forbidden). However, a small minority of Shia do perpetuate the practice in defiance.

Tatbir is practiced by some Shia Muslims on the 10 Muharram of the Islamic calendar, known as "the Day of Ashura", and on the 40th day after Ashura,

Known as "Arba'een/Chehelom" by Twelvers (the largest branch of Shia Islam, with about 85% of all Shias) around the world, some Shia may also perform tatbir on other occasions as well.

The practice of Tatbir includes striking oneself with a form of a talwar "sword" on the head, causing blood to flow in remembrance of the innocent blood of Imam Husayn. Some Twelvers also hit their back and/or chest with blades attached to chains.