The Legendary Dragon's are bosses in Sword Art Online and New ALfheim Online. They are fused of the Dragon Signers.
To summon a Legendary Dragon, the Signer must have defeated it previously (though there are exeptions, where the dragon chooses his partner, such as Galant's case).
After this, both beings fuse into one. As result of this "fusion", the player earns a red mark of the dragon on one of their forearms. The Legendary Dragon and his Signer are extremly connected to each other. Sometimes, the Dragon is manifestation of the true deep true personality and the abilities of the Signer. The player also gains many abilites of the Dragon. For instance, Galant's "Stardust" Sword Skills or Aki's Demeter's Gift.
The player can call upon his dragon whenever he wants to. First, the mark begins to shine red. Afterwards they need to recite a special chant for each specific dragon. Each chant has a close reference to the powers of the flying beast. After this, the dragon is summoned.
It should be noted that for the summon, it's necesary great power and energy. The souls of the Dragon and the Signer are so connected to each other. Any darmage caused to the dragon affects the Signer and vice versa.