The Bosses of Sword Art Online
The boss system in SAO involves a dungeon and a boss on every floor. There are 100 floors, meaning 100 bosses. However their are also special event bosses like Nicholas the Renegade, and because of the nature of the show, we don't see many of the boss fights. On this page I will be covering the bosses shown in the anime, and I will not be including Nicholas the Renegade because he is a flag mob. However I will be including the Fatal Scythe.
An example of a boss room.
Illfang the Kobald Lord
Illfng has four health bars and fights with an axe and sheild at first. When his last bar goes to a third, he instead uses a nodachi, which is the weapon he used to kill Diavel during the battle. He is supported by three Ruin Kobald Sentinels. Illfang is the boss of the 1st floor. Kirito finished him off, and gained the last attack bonus, the Coat of Midnight.
Illfang, with his four health bars shown, (above his left shoulder).
The Gleam Eyes
The Gleam Eyes was initially encountered by Kirito and Asuna when they were exploring the 74th floor dungeon. As soon as they saw him, they ran away from the boss room before running into members of the Liberation Army. although Kirito and Asuna begged them not to, the Army members entered the boss room, and were being devestated when Kirito and Asuna arrived with Klein. The arena was an anti-crystal trap, meaning the Army, couldn't teleport out. Kirito then activated his Dual Blades skill in a last ditch effort, and killed The Gleam Eyes with a sliver of health left.
The Fatal Scythe
The Fatal Scythe was a hidden boss in the dungeon below the Town of Beginnings. It was encountered by Krito, Asuna, and Yulier of the Liberation Army while they were going to save Thinker. Even combining their defenses, Kirito and Asuna were unable to fend off the massive scythe. Yui accessesd the system to save her "mommy and daddy", and killed the boss using the system. By then Yuilier and Thinker had teleported out, and the as the bos died, Kirito and Asuna said goodbye to Yui, who was caught by the system and returned into it.
The encounter with the Fatal Scythe.
The Gleam Eyes before the battle begins.
The Skull Reaper
The Skull Reaper was the boss of the 75th floor, and the first to have five health bars. This was the technical last real boss fight due to the game clear afterward(explained below in Heathcliff section). It proved far more difficult then anyone anticipated, even with all the best players, including Kirito, Asuna, Klein, Agil, and Heathcliff, fighting it. By combined effort it is defeated, but many of the players died. Part of their strategy involved defending its attacks together, and this time it was more successful then with the Fatal Scythe.
The Skull Reaper on the celing of its boss room before it drop attacked the players.
Heathcliff
Heathcliff, a.k.a. Kayaba Akihiko, created Sword Art Online. He then took the identity of Heathcliff and formed the Knights of the Blood Oath. He was going to eventually leave them and take his place as the 100th floor boss, but this was thwarted when Kirito discovered his identity after the fight with the Skull Reaper.
Round 1
The first battle with Heathcliff was a duel that happened because Heathcliff forbade Asuna to leave and go with Kirito. Kirito agreeded to dul for Asuna's freedom, but lost due to Heathcliff being protected by the system. Thus Kirito was forced to join the Knights of the Blood Oath.
Round 2
After Kirito discovered Heathcliff's true identity on the 75th floor, Kayaba proceeded to paralyze everyone except Kirito. He decided he was going to head directly to the Ruby Palace on the top floor, but before he did, he was going to give Kirito a reward for discovering the truth. The reward was another duel. Heathcliff tricked Kirito into activating the Eclipse, so that he could break Dark Repulser. When he was about to kill Kirito, Asuna jumped in the way, and took a killing blow. Shaken, Kirito tried to attack once more, but Heathcliff stabbed him, also killing him. Kirito's willpower however allowed him to barely stay alive, and he killed Heathcliff,clearing the game.
Kirito vs Heathcliff, 1st round.
Heathcliff vs Kirito, Round 2.
Round 3 (Speculation/100th Floor)
We can only speculate what a fight with Kayaba might have been had Kiritonot cleared the game early. Perhaps he would have turned himself into a boss creature of some sort. Perhaps he would be utilizing every sword skill availible. Regardless, Kirito defeated Heathcliff before the Ruby Palace, and brought the death game to an early close.
Heathcliff as shown in the theme song.