Ardyn's dagger is a a knife with a sharp point and two sharp edges used as a stabbing weapon kept by Ardyn Izunia in the Final Fantasy XV Universe. He appears to have it concealed upon his person, rather than summoning it with his special powers like he does with the weapons he wields in battle.
Where the dagger originates from is unknown, nor is Ardyn shown drawing it; rather, the dagger appears in his hand for the scenes he is about to use it.
As depicted in Final Fantasy XV: Episode Ardyn, after being released from this two-thousand-year-long captivity and taken to Niflheim, Ardyn ends up daemonifying the Astral Ifrit and the subsequent glimpse into the deity's psyche sends Ardyn into a hallucination of his own making where he imagines meeting his fiancée Aera Mirus Fleuret and his treacherous brother, Somnus. Aera deems herself culpable for Ardyn's fate and begs to be punished. Ardyn suddenly realizes he is holding a dagger while an evil Somnus approaches and forces a reluctant Ardyn to kill Aera with it.* The experience drives Ardyn mad and he becomes consumed by lust for vengeance. After learning from If the player fails the quick time event when Somnus and Ardyn struggle, Ardyn will be forced to stab Aera. If the player succeeds, Ardyn casts the knife aside, but Somnus strikes her down with his Armiger.Bahamut his current role as a sacrificial lamb to end the Starscourge plague, he kills the illusionary visions of Somnus and Aera with the dagger before laughing hysterically as darkness spreads.
As shown in Final Fantasy XV, during Leviathan's Trial Ardyn murders Lunafreya Nox Fleuret, the current Oracle and Aera's distant descendant in the manner he had envisioned in his hallucination, by stabbing her with his dagger. She perishes soon after while protecting Prince Noctis, Somnus's distant descendant. Noctis is left unconscious on the remains of the Altar of the Tidemother, where he is found by his friend Ignis Scientia. In Final Fantasy XV: Episode Ignis, Ardyn is shown threatening the unconscious Noctis with the dagger, but is stopped by Ignis.
Ardyn is not seen with the dagger afterward.