Thunderseal is Ky Kiske's weapon from Guilty Gear series. It also known as the "Fuuraiken" ( 封雷剣) 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 his 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 in different speed and strength.
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 the Thunderseal was sacrificed as a power source for the time freeze seal on the Maiden of the Grove. Ky Kiske then used another sword called Aquila.