"Sozin's Comet" is the four-part series finale of the American animated fantasy action Nickelodeon television series Avatar: The Last Airbender, created by Michael Dante DiMartino and Bryan Konietzko. It was written by the creators alongside Aaron Ehasz, and directed by Ethan Spaulding, Giancarlo Volpe, and Joaquim Dos Santos. Although the finale is split into four episodes, it aired as a two-hour four-part film on July 19, 2008. The Saturday airing of "Sozin's Comet" acted as a climax to a week of ten new episodes that concluded Avatar's third season. Before the week of July 14–19, no episodes had been shown in the US since November 30, 2007, though some episodes had been released on DVD prior to their airdate. This is also considered a 92-minute television movie.

The finale focuses on series protagonist Avatar Aang's non-violent personality and his reluctance to kill Fire Lord Ozai, as shown throughout the series. The finale also follows the exploits of many of Aang's friends and allies, including Sokka, Toph and Suki's struggle to destroy a Fire Nation airship armada, Zuko and Katara's battle against Azula, and Iroh and the Order of the White Lotus' attempt to liberate the city of Ba Sing Se.

"Sozin's Comet” was critically acclaimed from critics and fans alike. The initial showing averaged 5.6 million viewers, a 195% increase compared to the previous year in its time period. The premieres of episodes beginning with "The Western Air Temple" throughout the week received over 19 million viewers, of which 5.6 million were from Sozin's Comet. At the 2008 Annie Awards, the director of the third part of "Sozin's Comet" won an award for "Best Directing in an Animated Television Production".

Although "Sozin's Comet" is the series finale, Avatar: The Last Airbender has continued as an ongoing graphic novel series published by Dark Horse Comics since January 2012, which takes place after the events of the finale and before the sequel series The Legend of Korra.

With Sozin's Comet just days away, Prince Zuko pressures Avatar Aang, Katara, Sokka, Suki, and Toph Beifong to prepare for their fight against Fire Lord Ozai, only to learn that they intended to attack after the comet's passing. Zuko reveals that just before his defection, he attended a war meeting where his father planned to use the comet's firebending-enhancing power to end the 100-year war with a genocide against the Earth Kingdom. Alarmed, the group begins last-minute training; however, Aang can't bring himself to kill Ozai, as it goes against the culture that he is the last remnant of. That night, he sleepwalks onto a floating island, resulting in his friends scrambling to find him. Meanwhile in the Fire Nation, Princess Azula is barred from participating in the genocide and is appointed to a ceremonial Fire Lordship as Ozai promotes himself to a global monarch. Still hurt by her friends' betrayal and recalling her perceived abandonment by her mother, Ursa; Azula spirals into insanity and banishes many of her aides.

After failing to recover Aang, his friends seek Zuko's estranged uncle, Iroh, finding him in a secret society alongside their past mentors. Zuko reconnects with Iroh and encourages him to fight Ozai, but Iroh states that it is Aang's responsibility alone. Once the comet arrives, the gang split up. Sokka, Suki, and Toph sabotage the genocide fleet, the secret society liberates Ba Sing Se, and Zuko and Katara confront Azula. Azula duels and defeats Zuko, but Katara heals him after fighting and trapping Azula. On the island, Aang confers with past Avatars on how to defeat Ozai without violating his pacifistic values, but their input dissatisfies him. Eventually, he discovers that the island is actually a Lion Turtle, which teaches him how to bend people's chi. Aang is dropped off on the Earth Kingdom's shores, where Ozai confronts him.

Aang and Ozai battle, but Aang still can't bring himself to kill his foe. When Ozai accidentally unlocks Aang's blocked chakras by striking his back scar, the latter enters the Avatar State and prepares a death blow, only to stop himself. Remembering the Lion Turtle's lesson, he permanently depowers Ozai by bending his chi, ending the war. Some time later, Zuko is crowned Fire Lord and vows to help rebuild the world. Iroh reopens his tea shop in Ba Sing Se, where Aang and Katara share a kiss.

Co-creators Michael Dante DiMartino and Bryan Konietzko wrote the majority of the finale; DiMartino wrote parts one, three, and four, and Konietzko assisted with parts three and four. Aaron Ehasz, co-executive producer and head writer of the show, wrote part two. Ethan Spaulding directed part one, Giancarlo Volpe directed part two, and Joaquim Dos Santos directed parts three and four. Dos Santos was nominated for an Annie Award for his directing of part three, "Into the Inferno". Although "Sozin's Comet" was originally written as a three-part story, the creators noticed that the length had grown beyond what they had predicted from the initial script. To avoid pacing issues, they split the final part in two, adding several scenes to fill the remaining time.

The cast of "Sozin's Comet" includes all of the key characters from season three. Protagonists Aang, Katara, Sokka, and Toph are voiced by Zach Tyler Eisen (in his final role before his retirement), Mae Whitman, Jack DeSena, and Jessie Flower respectively. Iroh is voiced by Greg Baldwin and Zuko is voiced by Dante Basco. Grey DeLisle and Mark Hamill voice the two antagonists, Azula and Ozai, respectively. The co-creators also lend their voices to two minor characters in the third part, "Into the Inferno".

The animated effects in the finale were directed by Jae-Myung Yoo. A particularly tricky part to animate was the climactic scene where protagonist Aang tries to overpower antagonist Ozai. Konietzko originally pitched the scene to DiMartino and Ehasz, but the three of them encountered difficulties communicating how the scene would look. They ended up drawing a fully colored storyboard to send to the animation studios. Additionally, the finale featured a significant amount of fire compared to previous episodes, and were all hand-drawn by Yoo's team. Dos Santos observed that the fire effects alone cost more than most shows spend on an entire season. Although Avatar is not considered anime because of its American origin, Tasha Robinson of the SciFi Channel observed that "Avatar blurs the line between anime and [US] domestic cartoons until it becomes irrelevant." An IGN reviewer commented that "Sozin's Comet" "had that classic anime-look that I've always loved to see when watching old anime movies. The story through the animation was perfectly done."

The finale's music was written and composed by "The Track Team", led by Jeremy Zuckerman. Unlike past episodes, a music track produced by a live orchestra was used, rather than one in MIDI format. According to Konietzko, one of the most challenging scenes to pair musically was the Agni Kai duel between Zuko and Azula. The scene's pairing of moody and melancholic music with intense action was inspired by Ghost in the Shell, and a decision was made to quiet the sound effects to let the music set the tone, inspired by Blade Runner.

Avatar: The Last Airbender borrows extensively from East Asian art to create its fictional universe. Four "bending" arts exist in the universe; they are based on different styles and variations of Chinese martial arts: Baguazhang for airbending, Hung Gar and Southern Praying Mantis for earthbending, Northern Shaolin for firebending, and tai chi for waterbending. The fight scenes were choreographed by Sifu Kisu, who performed and filed every fight sequence with Konietzko to serve as reference for the animators. Additionally, the finale borrowed heavily from the Taoist concept of balance and order. The Avatar, an incarnation of a divine being, is supposed to maintain the world's order.

Nickelodeon originally broadcast "Sozin's Comet" from 8:00 pm to 10:00 pm EST on July 19, 2008. Just ten days later, on July 29, the "Book 3: Fire – Volume 4" DVD was released, which contained the four episodes as well as episodes 56 and 57, audio commentary from the series' co-creators, cast, and crew, and a comic book.

"Sozin's Comet" received critical acclaim; Ed Liu of Toon Zone stated that it made Avatar "one of the finest animated television series ever made", Andrew Whalen of Newsweek called it a "rare accomplishment", Ed Liu of Toon Zone praised the skill of the animation directors in designing the sweeping movements of the battle sequences, as well as the slower scenes, "including one moving reconciliation and the quiet coda that ties off many of the remaining loose ends of the series". Tory Ireland Mell of IGN praised the artistic design, stating that the "whole dark tone was gorgeous to look at". Mell also praised the writing, saying that the "story moved and kept us moving right along with it from beginning to end". She gave the episode a ten out of ten "Masterful" rating. Susan Stewart, reviewing for The New York Times, praised the show's effort at philosophizing, noting that "this is complicated stuff, the reconciliation of religion and violence, and it's beautifully rendered: simple enough for Nickelodeon fans and subtle enough for their parents, with humor to puncture the pomposity inherent in the heroic genre."

Reviewers remarked on the difficulty in understanding the voice of a new character: the Lion Turtle. The Lion Turtle was designed by Bryan Konietzko and character designer Jae Woo Kim. Konietzko disliked how the Lion Turtle turned out; he felt that the art was not up to standards of the original design he had received.

The premiere of "Sozin's Comet" averaged 5.6 million viewers, 195% more viewers in its time period than Nickelodeon had received in the previous year. During the week of July 14, 2008, it ranked as the most-viewed program for the under-14 demographic. The premiere of episodes 52–61 throughout the week of "Sozin's Comet"'s release received a total of 19 million views, and Avatar earned four spots in Nielsen's list of Top 20 Cable Results for the week ending July 20, 2008. It also appeared on iTunes' top ten list of best-selling television episodes during that same week. "Sozin's Comet"'s popularity affected online media as well; "Rise of the Phoenix King", a Nick.com online game based on "Sozin's Comet", generated almost 815,000 game plays within three days. A video game loosely based on the third season, Avatar: The Last Airbender – Into the Inferno, was released on October 13, 2008.

The finale episodes were also recognized by the awards circuit. Joaquim Dos Santos won the "Best Directing in an Animated Television Production" caption in the 2008 Annie Awards for his direction in "Into the Inferno". Additionally, music editor and composer Jeremy Zuckerman and the sound editing team were nominated a Golden Reel award for "Best Sound Editing in a Television Animation" for their work in "Avatar Aang".