This skill, Chain Lightning, is property of Ishimura Elite.
Chain Lightning is a special skill available to only highly skilled and trained players of Sword Art Online. It is unique in that it changes depending on the character and their play style.
The exact nature of Chain Lightning is player determinant, however it is always lightning elemented, and will affect multiple enemies with lightning arcing from one enemy to the next. The effects Chain Lightning has on enemies can be from stuns to debuffs to outright damage, and the nature of the arcing effect can vary greatly depending on the player.
Ishi - Ishi's Chain Lightning is among the most powerful. It is used from Gilgamesh, which from the start increases its power, and requires physical contact, or close enough to it. Ishi's Chain Lightning is meant to deal extreme damage to multiple enemies instantly. The arcing effect is instantaneous among a crowd of enemies, appearing as one bright blue lightning bolt strung between each enemy that quickly disappears. However, the damage this does in such a short time is incredibly high, being able to instantly destroy most enemies, while higher level enemies and bosses that can survive it will most likely be stunned for a while by the powerful and impacting shock. Ishi is able to control which enemies to hit among a crowd, adding precision to this already powerful skill, and is able to adjust the power of the attack to lower or heighten damage dealt. At higher levels of power it can be used at range.
If used at maximum power, this attack will backlash on Ishi, causing considerable damage to himself, but dealing innumerable amounts of damage to his target, enough to almost assuredly destroy any target, even levels above him.