Boss

A Boss, in video game terminology, is a unique computer-controlled monster or character that is much more powerful than an average monster or non-player character encountered in the game until that point and usually serves as a challenge to the player at the end of particular section of a game, usually a level or stage, or guarding a specific objective.

In the Sword Art Online Universe
Throughout the Sword Art Online series, there are several types of bosses that can be encountered: Floor bosses, Field bosses and Flag Mobs. Most, but not all, bosses within the series, especially Floor Bosses, have “the” in their name to signify their uniqueness as bosses.

Floor Boss
A Floor Boss (フロアボス), alternatively called a Labyrinth boss, is a type of boss that can only be encountered in a large room, called the boss room, on the 20th floor of a Labyrinth in the castle of Aincrad. A Floor Boss’s main purpose is to guard the entrance to the next floor, requiring players to defeat it to advance towards the 100th Floor. Each Floor Boss is a powerful creature, with several HP bars, that usually requires a raiding party of 48 players or more to defeat and usually has special attack patterns that are activated once the monster’s HP falls below a certain level. Some Floor Bosses also have minions that are constantly summoned throughout the battle to aid it, though some bosses do not have minions at all. Floor Bosses also never leave the rooms they reside in.

The difficulty of each Floor Boss varies from floor to floor, with some causing little to no trouble, like the boss of the 22nd Floor, while other, especially the bosses on Aincrad’s quarter floors, are extremely dangerous and have led to numerous deaths in Sword Art Online. The boss rooms of the 74th and 75th Floors are also Anti-Crystal Areas, and it can be presumed that the remaining floors also host such effect.

Floor Bosses are also present in New Aincrad in New ALfheim Online, however, all of the Floor Bosses of the original Aincrad were replaced with new bosses that, due to ALfheim Online not being a death game, are usually much more powerful than their counterparts in the original Aincrad. Floor Bosses within New Aincrad also have their HP bars hidden from the players to make the battles even harder by making it harder to judge how much HP the boss has remaining.

Known Floor Bosses

 * Illfang the Kobold Lord (1st Floor Boss)
 * Baran the General Taurus (2nd Floor Boss)
 * Wadjet the Flaming Serpent (Aincrad version 8th Floor New Boss)
 * Two-Headed Giant (unnamed 25th Floor Boss)
 * Metallic elemental (unnamed 27th Floor Boss)
 * The Four-Armed Giant (unnamed New Aincrad version 27th Floor Boss)
 * Six-armed metallic Buddah statue (unnamed 50th Floor Boss)
 * Armoured stone warrior (unnamed 60th Floor Boss)
 * The Gleam Eyes (74th Floor Boss)
 * The Skull Reaper (75th Floor Boss)
 * Heathcliff (supposedly, the 100th Floor Boss)

Field Boss
A Field Boss (フィールドボス) is a named monster strategically placed at the entrance to a Labyrinth on most floors in Aincrad. The main function of this Boss is to prevent access to the Labyrinth of the floor until the monster is defeated. A Field Boss's den is usually surrounded by cliffs or ranging streams on both sides, arranged in such a way that there would be no way to access the Labyrinth until the Boss is defeated. Like Floor Bosses, each Field Boss can only be defeated once, however, they are much easier to defeat and can be defeated by a single party, if all members are sufficiently leveled and have enough experience in battle.

Not all floors have a field boss, for example the 1st Floor and the 22nd Floor did not have any, while some floors might have more than 1.

Known Field Bosses

 * Bullbous Bow - |2nd Floor
 * Geocrawler - 56th Floor

Flag Mob

 * A Flag Mob (フラグ Mob) is a monster set up for the completion of a quest or event. These type of monsters can spawn between every few hours to every few days, while some Flag Mobs only spawn once a year or do not re-spawn at all. Some rare Flag Mobs that are usually as powerful as a Boss monster usually also drop rare and valuable items upon being killed while other Flag Monsters are barely more difficult than a regular monster. Some Flag Mobs are part of quests that have sub-routes, thus a certain event may be triggered during the quest that would help defeat the boss, for example, taking a certain sub-route may lead to a temporary NPC partner for the quest.

Known Flag Mobs

 * Nicholas The Renegade - Christmas event boss that drops a revival item called Divine Stone of Returning Soul.
 * Lungfish - a fishing event boss that drops a rare fishing rod.
 * The Witch of the West - a boss that is required to be killed for the «Witch of the West and the Three Treasures» quest.
 * Þrym - the boss required to be killed for the completion of the Excaliber quest.