Refers to someone who endures great suffering, often for a cause or belief. Can be used in religious contexts (martyr) or more generally for a person who suffers.
He is known as a martyr who gave his life for his faith.
It is painful to hear the stories of the sufferers of war.
被害者 means 'victim' (of a crime, accident, etc.), focusing on being harmed by an external event, whereas 受難者 emphasizes enduring suffering, often with a sense of passive endurance.
Compound of 受難 (suffering, passion) + 者 (person). The term 受難 is often associated with the Passion of Christ, giving the word a religious connotation, but it is also used in secular contexts.