In the Extended continuity, Longinus is Kiros‘ canon Dragoon weapon. A spear of presumably elven make, Longinus was found during the exploration of Laxan Loft some few years before the events of Shadowbringers. Just like Excalibur and the Aegis Shield for Hikaru, the Longinus is attuned to Kiros specifically, matching its strength to his own. The spear additionally serves as the soul crystal for the Dragoon job.
Longinus is notable for its spearhead, which is made of a pair of entwined blades around another pronged blade. The spear surges with intense dark aether, flaring and vibrating in the presence of certain enemy types; initially, it reacted only to powerful Sin Eaters, but during Endwalker, Longinus displayed the same behavior when near Blasphemies and voidsent as well. When used against either of these foes, Longinus is capable of breaking them down as if it were breaking down their bodies on a molecular level, often times far faster than if an appropriately-aspected weapon or spell were used instead. Though this was initially attributed to it being dark-aspected, a more detailed analysis later in Endwalker determined that the “unmaking” effect is actually Dynamis-related and ignores the elemental affinity of its target, with the potency being affected by the wielder’s emotional state.
During much of Shadowbringers, Kiros is not able to wield Longinus (or use the Dragoon job) as he used it to grant his elder sister Hikari a swift death to prevent her from becoming a Sin Eater. The sight of the spear made him nauseous, and thus he kept it wrapped in thick cloth. It was only after slaying Forgiven Narcissism, the Sin Eater that his father became, that Kiros was able to wield Longinus again.
Kiros specifically requested to be placed on the front lines for Vanaspati because Longinus reacted strongly to the presence of Blasphemies, especially Svarbhanu. Taking a gamble on Longinus potentially having the same lethal effect on Blasphemies as it did on Sin Eaters, Kiros used his Limit Break Blade Destroyer on Svarbhanu before the Blasphemy was about to loose a lethal hail of comets onto the battlefield. As predicted, Longinus rapidly chewed through the Blasphemy’s flesh. During Endwalker postgame, Longinus would prove highly effective against the voidsent of the Thirteenth, including the Archfiends themselves.
Because Kiros himself is dark-aspected, and many of his Dragoon weaponskills deal dark-aspected damage, Longinus is especially powerful in his hands. Putting aside its inherent effectiveness against Sin Eaters, Longinus does massive damage to light-aspected enemies as a whole.