Nick Buoniconti is an NFL legend. He was a defensive star on the 1972 Miami Dolphins, still the only team to go undefeated and win the Super Bowl in the same season.

Buoniconti, of course, is still a legend -- legends never die, as the saying goes. But he's declined precipitously over the past few years, following 14 seasons of big hits and playing through pain as a pro football demigod.

That decline -- equal parts harrowing and heart-breaking -- is the focus of a long, painstakingly intimate profile of Buoniconti in Sports Illustrated, written by S.L. Price. You may have read more concussion stories than you can count at this point, but it's time to make room for one more.

The strength of the piece is in the layers of Buoniconti's life that Price peels back. We see Buoniconti's struggles, but also those of former teammates suffering through a similar decline. We also get a visceral sense of the pain inflicted upon Buoniconti's family -- including his son Marc, who was paralyzed while playing a college football game for The Citadel in the 1980s.

Buoniconti excelled in his post-football careers -- as a sports agent, a U.S. Tobacco exec and later spearheading a foundation aimed at curing paralysis. But by the time he first calls Price to share the story of his physical and cognitive declines, Price overhears Buoniconti asking his wife, Lynn, to remind him how to hang up a phone.

Buoniconti also needs help from Lynn when peeing, has bouts of irrational anger, and regularly falls on his face while walking.

"I feel lost," he tells Price. "I feel like a child."

This video shared with Sports Illustratedby the family shows him struggling simply to remember how to put on a T-shirt.

Buoniconti, who approached Price about the profile in hopes of helping other former players, began suffering from symptoms in 2013.

Doctors today believe he's afflicted by a range of ailments including chronic traumatic encephalopathy, or CTE — a degenerative brain disease associated with repeat concussions and found in an eye-popping number of former NFL players. (CTE can only be definitely diagnosed via autopsy, however.)

Buoniconti tells Price he feels fortunate to be financially secure and to have a companion like Lynn caring for him. He also says he has compassion for other players suffering similar fates amid less fortunate circumstances.

Lynn, meanwhile, briefly illuminates her own struggle while being a rock for the man she married.

"When you marry your best friend and now he’s not your best friend anymore because there’s someone else in there, it’s very difficult," she says. "He looks like the person I married. But he really isn’t there."

You can read the full profile at SI.com -- it's well worth your time.