Songfacts®:

• Slayer guitarist Jeff Hanneman passed away in May 2013 from alcohol-related cirrhosis of the liver. The album's title track was written by co-shredder Kerry King as a tribute to his longtime bandmate. "I call it the Hannemanthem! I wrote that for Jeff," King told Metal Hammer.
• Kerry King said about the writing of the song: "I went with it based on Slayer. Slayer is repentless; Slayer's always been repentless. I said, 'You know what, I should write something about how I think Jeff looked at the world, because he looked at the world exactly how Slayer looked at the world.' Plus little 'Jeff-isms.' It was both easy and difficult. The difficult part was doing it justice; I didn't want to just throw it together. I wanted it to be solid, as deep as it could be without getting personal."
• Slayer performed this live for the first time at the opening show of the 2015 Rockstar Energy Drink Mayhem Festival on June 26, 2015 at Sleep Train Amphitheater in Chula Vista, California.
• Asked by Artist Direct why Slayer named their album after this cut, Kerry King replied: "That song wasn't written to be the title track. It just happened that way. I remember one time at rehearsal Paul (Bostaph drums) and I got done playing that song, and I was like, 'Dude, that song is f--king relentless.' Soon thereafter, I said, 'Repentless,' not knowing it wasn't a word! I looked it up, and I was like, 'Are you kidding me?' [Laughs] Sure as s--t it's not a word."
  
  "The funny thing is if somebody makes up a word that's not a word, you usually have to ask, 'What does it mean?' When you hear the word Repentless, I think 90 percent of the people know exactly what it means. It was very odd for me to find out that wasn't a word."
• The song's music video about rioting prisoners features an array of iconic character actors, including Heat and Con Air 's Danny Trejo and Tony Moran, who played the unmasked Michael Myers in the original Halloween, Derek Mears (Predators, Friday the 13th), Tyler Mane (Halloween), Jason Trost (Hatchet III), Sean Whalen (Men In Black, Halloween II and Vernon Wells (Mad Max 2, Commando, Weird Science) also feature.
  
  The clip was filmed by Hatchet III director and Avengers: Age of Ultron cameraman BJ McDonnell and filmed in the Sybil Brand Institute, a former county jail for female inmates. The stunts were co-ordinated by Nils Allen Stewart, who is known for his work on films such as Planet of the Apes, Anger Management and Daredevil.