Bleach , titled "Koga's Rebirth" (or "Kouga to be Revived!!!"), is part of the Zanpakuto Unknown Tales filler arc. It originally aired on November 24, 2009. Episode Summary
It is revealed that Muramasa’s primary goal was never to liberate the Zanpakuto, but to use Yamamoto's spiritual pressure to break the seal on his master, Koga Kuchiki.
Ichigo, Kenpachi, and other Soul Reapers attempt to break into the cave where Head Captain Yamamoto is being held.
In this episode, the conflict against Muramasa and the manifested Zanpakuto spirits reaches a critical point as the true antagonist, , is revived.
Successfully manipulates the Zanpakuto spirits to achieve his master's freedom.
Appears to be siding with Muramasa, though his true intentions remain a focal point of mystery during this arc.