Léviathan is a non-player character from the ALfheim Online virtual world who appears in the 2013 anime special Sword Art Online: Extra Edition. His background is that of a quest-related entity encountered during the Deep Sea Plunderers quest, a mission the main characters undertake to retrieve an item called the Child's Egg. He is depicted as a giant man, several times the size of a fairy avatar, with short gray hair, a short gray beard and mustache that connect with his hair, thick gray eyebrows, and a serious face marked by wrinkles. He wears silver-colored armor and a large blue cape.

Léviathan's personality is not explored in depth; his demeanor is stern and imposing, and he does not engage in extended conversation with the player characters. His motivations are implied by his role in the quest: after the party defeats the quest's boss, a kraken known as the Abyss Lord, Léviathan appears and takes the Child's Egg. This suggests he functions as an overseer or guardian of the quest's objective, but his specific reasons for claiming the egg are not explained. In the story, he serves as a brief narrative device that concludes the underwater questline, adding a final twist to the party's efforts.

Léviathan has no known relationships with other characters, as he interacts with the group only in that single scene and does not appear again. He undergoes no development within the special. His notable abilities are implied by his large stature and armored appearance, which suggest high physical strength and durability, though no specific combat or magical skills are demonstrated. Overall, Léviathan is a minor but memorable figure whose intervention marks the end of the Deep Sea Plunderers quest.