Scar has long been one of the most iconic villains in the history of Disney movies, murdering Mufasa and taking over Pride Rock in The Lion King. That said, no one has ever known what made the character so evil in the first place, nor how he got that mark on his eye and took on the infamous mantle of Scar. Things changed when the Disney Junior series The Lion Guard wrapped up this weekend, and Scar came back in the form of a spirit to explain his backstory.
Having died in the original Lion King film, Scar's spirit was somehow preserved in a volcano near Pride Rock, and he returns to reveal some history regarding the Lion Guard, of which he was in charge in his younger years. He was a bit of a loner while on the Guard and often took patrols by himself, which leads to exactly how the scar came about.
While patrolling the area surrounding Pride Rock, Scar ran into another lion, one who had plans of overthrowing Mufasa's rule and taking the kingdom for himself. He enticed Scar with stories of them controlling the Pride Lands together, but a tall tale wasn't enough of a guarantee for the lion, so he tricked Scar into working with him. There was a snake hidden beneath a rock and the lion led Scar to it. The snake jumped out and bit him on his eye, causing the scar.
The lion told Scar that there was venom in the snake's bite and that he would take it out, but only if Scar agreed to help him and do whatever he said. This didn't stand with Scar and he ended up killing both the lion and the snake. That was just the story of the wound itself, and it was actually Mufasa who started calling his brother Scar.
When Scar went back to Pride Rock to say that he had taken out a mutiny before it began, he thought he would be praised for his work, but that wasn't the case. Mufasa simply laughed and called his brother Scar, instantly moving on to more important matters. That dismissal is what drove Scar to decide that his brother should no longer be king, and the plot to murder Mufasa officially began.
All it took was one simple patrol to turn Scar into the leader of The Lion Guard into the biggest villain Pride Rock has ever known.