In the world of Avatar: The Last Airbender, the Avatar is set apart from every other person in that they can not only bend all four elements, but can enter something known as the Avatar State. Fans of the franchise are very familiar with the Avatar State, since Aang has often demonstrated some of his strongest bending while in this legendary mode.
While it seems like he's entered the Avatar State dozens of times over the course of the series, he's actually only done so a handful of times, which is why the Avatar State still possessed such a huge degree of reverence throughout all three seasons. Below is every single episode in which Aang entered the Avatar State, often due to great emotional stress that led to moments fans will never forget.
10 The Avatar Returns (Book One, Episode 2)
In the very second episode of the series, Aang showed just how powerful the Avatar State was when he entered it for the first time after he was thrown into the ocean during his battle with Zuko. Despite only knowing Airbending at the time, he was able to tap into powerful Waterbending techniques from his past lives, and knocking Prince Zuko and some of his men off of their own ship with just one move.
It was here that fans realized that there were hidden depths to Aang's powers that they had yet to explore.
9 The Southern Air Temple (Book One, Episode 3)
When Katara freed Aang from the iceberg, he was blissfully unaware of the Hundred Year War and the genocide of his people. In "The Southern Air Temple", he returned to the place he was raised, and while Katara and Sokka knew that they wouldn't find any Airbenders there, Aang remained hopeful until he stumbled upon the corpse of his former master, Monk Gyatso. He entered the Avatar State after realizing what the Fire Nation had done.
This transformation into the Avatar State was one of the most significant in the series. It caused Avatar Statues and temples around the world to light up, signaling his return to the whole world, not to mention the fact that it was the first of many times Aang entered the state due to intense rage, making him a danger to anyone around him, whether they be friend or foe. Luckily, Katara was able to calm him down, but his anger was definitely justified.
8 Winter Solstice: Part 2: Avatar Roku (Book One, Episode 8)
When Aang traveled to the Fire Temple to talk to Avatar Roku, he found himself in a tough spot after Admiral Zhao's forces lined up outside the sanctuary doors, preparing to fry him as soon as he stepped out. Roku offered his aid, channeling himself through Aang's body and using his powerful Firebending techniques to ward off Zhao's forces so that Aang and his friends could escape.
If only Aang were able to do this a lot more frequently, he could've taken Ozai on much earlier than he did.
7 The Storm (Book One, Episode 12)
"The Storm" gave backstory on Aang's life before he wound up in that iceberg, and explained how he ended up frozen for 100 years in the first place.
He nearly entered the Avatar State out of anger when he recalled the fact that the Southern Air Temple's Council of Elders planned to send him away to resume his Avatar training away from Monk Gyatso, but entered the Avatar State fully when him, Katara, Sokka, and a fisherman nearly drowned during a storm. Using this power boost to his advantage, he created a sphere of air that he used to safely steer Appa from under the water and out of the storm.
6 The Siege of the North, Parts 1 & 2 (Book One, Episodes 19-20)
"The Siege of the North" was the heart-stopping conclusion to Book 1, and showed fans just what Waterbenders were capable of, but even before they lost the Moon Spirit, the Northern Water Tribe's forces were outnumbered by the Fire Nation's colossal armada. When Aang entered the spirit world to seek advice from Koh, he went into the Avatar State, as his eyes and tattoos glowed during his meditation.
He later went back into the Avatar State after the Moon Spirit was murdered by Zhao, resulting in one of the franchise's defining moments as he fused with La, the Ocean Spirit, and became a creature composed of light and water, commonly referred to by fans as "Koizilla". He took out dozens of ships and Fire Nation soldiers with the help of a spirit that otherwise would've been powerless, proving that the Avatar really was the bridge between the spirit world and the physical world.
5 The Avatar State (Book Two, Episode 1)
After hearing about what Aang did at the North Pole, Earth Kingdom General Fong and his forces attacked Aang and his friends in order to trigger the Avatar State. He was successful, much to the detriment of his men, many of whom were hurt as a result of Aang's indiscriminate fury while in this state.
General Fong might've been crazy, but he deserves props for figuring out just how to push Aang to this level.
4 The Desert (Book Two, Episode 11)
After Aang found out that Appa was kidnapped, he could barely control his emotions. After running into the very Sandbenders who stole Appa, he went into the Avatar State, destroying their sand-sailers and nearly destroying them!
If not for Katara calming him down, there's a very good chance that those Sandbenders might not have lived through their encounter with the Avatar.
3 The Crossroads of Destiny (Book Two, Episode 20)
In order to master the Avatar State, Aang trained with Guru Pathik, but he refused to finish his training because he just couldn't let go of Katara. During the battle against Zuko and Azula in the Crystal Catacombs of Ba Sing Se, Aang managed to go into the Avatar State after isolating himself from Katara and temporarily forgetting about her.
Fans were prepared to see him do some real damage, but Azula took advantage of the opening and shot him down, nearly ending the Avatar Cycle for good. It was this defining moment that left him with a nasty scar for the rest of his life, not to mention the fact that it blocked his chakra, making him unable to enter the Avatar State for the majority of Book 3.
2 The Avatar & the Fire Lord (Book Three, Episode 6)
While Aang didn't go into the Avatar State much in Book 3, he did go into it briefly in "The Avatar and the Fire Lord" when he meditated on Roku's island in order to convene with his past life in the spirit world.
1 Sozin's Comet, Part 4: Avatar Aang (Book Three, Episode 21)
The last time Aang went into the Avatar State came in the later half of his fight against Ozai. After being overwhelmed by Ozai's relentless Firebending, Aang's wound was struck, reopening his sealed chakra. He overwhelmed Ozai with some of the strongest bending fans had ever seen in the franchise, using all four elements in innovative, destructive ways never before seen. It's hard to believe that he didn't accidentally kill Ozai during all of this, which goes to show that even when he's in such a state, he still manages to remain true to his no-killing rule.
He briefly went into the Avatar State after taking away Ozai's bending in order to douse the singed Wulong Forest so that no one else would suffer due to Ozai's plot to burn the Earth Kingdom to the ground. After such a forceful display of the Avatar State's power, it was the perfect juxtaposition to show the state being used in a more calm, controlled manner, proving that Aang had finally become the fully-realized Avatar he was always destined to be.