Blade Master

Blade Master is a character "class" in Sword Art Online.

It was introduced into the game during Alpha Tests, but was soon discarded when SAO became a non-class based MMORPG. The files for Blade Master were retained, however, and remained unlockable to very few.

Overview
Although technically not a class at all, the Blade Master is widely regarded as a specialization in and of itself. Unlike other players, a Blade Master does not specialize in a single weapon or form of combat. Instead of this, Blade Masters learn to specialize in many bladed weapon types, from swords to scythes to halberds to axes. It is unknown what exactly qualifies being called a Blade Master, but a player who manages to become at least somewhat proficient with all blade types are elegible to be considered as a Blade Master, while those who have completely mastered all weapon types are fully considered true Blade Masters.

Completely mastering more than just two types of bladed weapons requires skill and ability that is rarely seen in players of Aincrad, and therefore the title of Blade Master is shared by only two individuals in SAO.

Blade Master Skills
There are two skills unique to only Blade Masters, and the conditions of unlocking either are completely random, but known to be difficult.

Blade Master's Creed 

A high level ability that grants complete mastery over all bladed weapons available in SAO, and also grants the ability to dual wield any combination of bladed weapons, not just swords (an oversight when Dual Blades was at first not a unique skill). This is a powerful passive ability that grants the user extreme and often imbalanced combat ability. Any bladed weapons that were already mastered at the time of obtaining Blade Master's Creed has it's proficiency level amplified further.

Blade Master's Promise 

Blade Master's Promise is the highest level skill available to a Blade Master, and is one of the most powerful skills in SAO, being attainable only after Blade Master's Creed has been achieved. Blade Master's Promise allows a Blade Master to wield any number of blades at once by having them float around the user. These blades are capable of independent movement and are capable of both attack and defense, assisting the Blade Master in both. This in effect makes a Blade Master practically unbeatable in combat.

Blade Masters in SAO
Ishi 

Considered the first and True Blade Master, Ishi is the first player of Aincrad to gain complete mastery of all bladed weapon types through Blade Master's Creed, and unlocked the highest level of ability attainable by a Blade Master through Blade Master's Promise. Ishi is widely considered as one of the most skilled players in SAO.

Asuka 

The second Blade Master, Asuka managed to gain proficiency with all blade types under Ishi's training. She eventually earned Blade Master's Creed, giving her extreme levels of combat ability, but was unable to achieve Blade Master's Promise like Ishi had.