From Granblue Fantasy Wiki
Xuanwu Seals are used to uncap water SSR weapons from Rise of the Beasts. They are also used to trade for Golden Beast Seals in the Shop.
Xuanwu Seals can be obtained by trading for them with event points during the Rise of the Beasts event.
- From Rise of the Beasts event store (10,000 first purchase, +5,000 for each additional purchase thereafter, capping at 30,000)