"To sum it up, he's a tough son of a bitch."
That's how WWE legend 'Stone Cold' Steve Austin describes his friend, colleague and former on-screen foe, Mark 'The Undertaker' Calaway.
The 55-year-old Calaway is the subject of a tremendous peek behind the curtain documentary, that is set for release on the WWE Network on Monday, May 11.
The five-part series, titled Undertaker: The Last Ride, documents the final three-years of the legendary sports entertainer's career.
The first episode, 'The Greatest Fear', deals with Calaway's 2017 WrestleMania match with Roman Reigns, which at the time he felt would be his final appearance inside the wrestling ring.
Viewers are granted a rare and revealing fly-on-the-wall perspective of Calaway's world at the time, which was full of nerves, self-doubt, lingering injuries and the adulation of his peers.
Three days before his match with Reigns, Calaway called WWE owner Vince McMahon to tell him Sunday would be his final match.
The two agreed to allow a WWE camera crew to document those final few days of what he thought would be the end of his career.
"WrestleMania - I mean that is just it. It is the crème de la crème of our industry and for me, it was just a big-time," Calaway says of what kept driving him to return year-in, year-out, despite barely being able to walk at times.
"It takes its toll. I had a five-year stretch there, where I had the same schedule. I would prepare for Mania - I would have my match - then I would have some kind of surgery to repair some sort of injury that had been bothering me before the match.
"Then I would go right into rehab and training to be ready to go at WrestleMania again."
The engrossing 55-minute journey features first-hand accounts with Calaway, his wife Michelle, McMahon, Austin, Shawn Michaels, Ric Flair and several other WWE stars.
The agony on wife Michelle's face when talking about Calaway's final push to get over the line in 2017, gives insight into what he put his body through for the fans and his peers.
"People have no idea what sort of pain he is in physically and how much pain he tolerates through all the surgeries he has had."
Calaway's love for the business he has dedicated his life to since the mid-1980s is obvious when vision shows the then 51-year-old struggling to walk down the entrance ramp, the night before his match with Reigns.
And it's clear his fellow entertainers respect him for all he did in paving the way for their success, from wrestler-turned-Marvel-star Dave Bautista to Olympic gold medallist Kurt Angle.
Former WWE champion John Layfield's honest assessment of Calaway's contribution to the business is telling.
"There is no doubt in my mind he is the greatest of all time - no doubt."
Subsequent episodes will deal with the aftermath of that night in Florida three years ago and how and why he kept going.
On the heels of ESPN's riveting Chicago Bulls/Michael Jordan inspired 'The Last Dance', the similarly titled WWE feature about the pro wrestling 'GOAT is a must-watch for the next five Mondays.