Tenshintai (転神体, lit. "Change [Death] God Body", Viz: "Divine Transfer Body"): Artifact belonging to the Onmitsukidō, but invented and tested by Urahara. The item is a vaguely human-shaped, man-sized doll used to forcibly materialize the spirit of a Zanpakutō into the real world when stabbed by the Zanpakutō. In doing so, the user can subjugate the spirit and attain the Bankai of their Zanpakutō. The materialization lasts for about three days, after which the spirit reverts back into the doll that spawned it. Without this invention, achieving the Bankai takes at least ten years, not to mention the many years of combat experience needed. This method, however, is very dangerous due to the fact that it forcibly materializes the Zanpakutō's spirit. If used more than three times consecutively, it is possibly fatal to the user
(Credits to BleachWikia)
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