Ky Kiske's signature weapon. Also known as the "Fuuraiken" (封雷剣, lit. "Sealing Thunder Sword") in Japanese, the Thunderseal is one of the eight "Jinki" or "OutRage" that the United Nations were keeping. When Ky Kiske rose in rank as the Holy Knights' commander, the Thunderseal was given to him as a recognition to his leadership and skill in lightning magic.
The exact capabilities of the Thunderseal are unspecified, but it is made specially for lightning magic users. Electricity can be charged in the blade and be used to make slashes and thrusts more deadly. Likewise, the electricity in the blade can be launched as projectiles at different speed and strength. Another unique function of the Thunderseal is its ability to create a subspace that effectively halts time itself, though Ky had no idea of this power and it was activated through desperation.
In Lightning The Argent, Sol Badguy confiscated the Thunderseal from Ky to stop him from interfering with his activities. He returned it before Ky faced Seevus. In Guilty Gear 2, Ky Kiske used the Thunderseal to stop time itself on the Maiden of the Grove to halt her sublimation, rendering it incorporeal. Ky Kiske then used another sword called Aquila. He later acquires a modified sword called Magnolia Eclair II, which greatly resembles the Sacred Treasure.
After Dizzy is released from stasis by Dr. Paradigm undoing the spell, The Thunderseal is restored to its original form and is sealed within Illyria Castle.
During the events of Guilty Gear -STRIVE-, Ky wields the Thunderseal once again, which has taken on a new appearance to accommodate for its wielder's increased power and skill.
- It is implied that The Thunderseal may be sentient. This is indicated by the blue sylph-like being that emerges from the OutRage when Ky executes his Instant Kill, Rising Force. Another hint is when it acted on its own to place the Time Chord on Dizzy in order to halt her sublimation and preserve her life.