This episode reveals how Scar obtained his scar and his namesake.
In addition, Ford Riley confirmed in a recent post  that Scar was originally named after his ancestor Askari, who was the leader of the original Lion Guard, but after getting his scar, he was shortened to the nickname of Scar.
This story contradicts the events of The Lion King: Six New Adventures book, A Tale of Two Brothers. The book states Scar's original name was Taka, Swahili for want or trash, and he had obtained his scar after being attacked by a herd of wildebeests. However, Six New Adventures, unlike the series, is confirmed not to be canon to the films or other spin-offs.
The story also contradicts Scar's backstory given in The Lion King (2019) (which ironically was released the day Season 3 wrapped up production) which implies that Scar got his scar after challenging Mufasa to a fight for the throne and Sarabi's paw in marriage. However, unlike, the 2019 remake, The Lion Guard is a sequel to the original film.
In 2017, Lion King director Rob Minkoff stated his belief of Mufasa giving Scar his scar when they were cubs. As this was stated by the director, it serves as the only canon contradiction to the series' explanation.
Kion obtains a scar, courtesy of Ushari, in a similar form of his great uncle.
Kion unleashes the Roar of the Elders at Bunga for the first time, as his deepest fear of becoming like Scar begins to come true.
In addition, it references the opening scene in the original film as the sun rises.
When Simba mentions that Pride Rock had survived fire before, he was referring to when he battled Scar for the throne back in the climax of the original The Lion King.
Before the Lion Guard leaves the Pride Lands to search for the Tree of Life, Nala voices the hope that the group might be back in time for Kiara's first hunt. This event is shown in the latter half of The Lion King II: Simba's Pride, with the Lion Guard notably absent. Here, it’s revealed that their absence was due to not having returned from their journey at the time.
Anga becomes the new keenest-of-sight, while Ono becomes the smartest. This marks the first time a Lion Guard has had 6 members.
This also marks the only time Scar appears at Pride Rock.
In regards to appearing in the Pride Lands instad of the volcano, the first time was in Divide and Conquer, the second time was in The Kilio Valley Fire, and the third time was in The Fall of Mizimu Grove.
When Scar is on top of Pride Rock, it is a reference to I Have A Plan on the lyric We’ll be glorious.
This is the final episode to have the title appear on the main screen.
It is revealed that Simba has accepted Kion's friendship with Jasiri.
This is the first time blood is shown in the show, as when Ushari initially bites Kion, blood can be seen on his scar (although he isn't bleeding). This is up until Kion exits the volcano after Scar's defeat, where his scar turns into a pink flesh-wound.
This marks Anga, Shujaa, Hodari, Kijana, Baby Baboon's Mother, and Baby Baboon's second appearances in the series.
This marks the final appearances of Reirei's Pack, Kiburi's Float, Shupavu's Group, Hodari, Wema, Tunu, and Ushari in the series. Although Scar's spirit is destroyed, he appears again in Friends to the End.
It is possible that Scar could've appeared as an hallucination to Kion, and not an official appearance as he appears in a reflection. Kion could be either be having delusions, or the venom has had an effect on him. Given that in the episode, Scar's lines are recycled from his final words, and he does not say anything else, it's likely that Scar's ghost didn't physically appear and Kion was hallucinating.
The special marks the first appearances of several characters: