Ragnarok

Ragnarok is a sword. The damage of Noctis' warpstrikes is boosted when warping with this weapon.

Source
Ragnarok can only be obtained through the FFXV Booster Pack + DLC.

Trivia

 * When warping with Ragnarok, Noctis glows red instead of his usual blue. The animation for the warp-strike is also slower than warp-strikes performed with other weapons.
 * Ragnarok is colloquially understood as the apocalypse in Norse mythos. It is a final battle between the Norse deities, culminating in the destruction of the old world and the beginning of a new one.