User blog comment:Jopjopjop/Volume 20 Spoilers/@comment-50.139.232.230-20150731205208/@comment-26285797-20150806210001

Longinus Smasher is the forbidden, final attack of the Boosted Gear. Great Red enabled Issei to use it during their battle with Jabberwocky, but this attack is inherent to the Boosted Gear. Issei unlocked this attack after Ddraig and Albion, with the help of Vritra and Fafnir, helped end the strife of the past possessors of Divine Dividing.