The third form of a
is Bankai or "full release" is the true form of the Zanpaktou and has immense power. For the most part only Lieutenant level and higher Shinigami have obtain this level because the imence time it takes to train to call out the true power of the sword. The powers, form, and limit, greatly depend on the soul, spirit energy, and personality of the wielder. But also depends on the sword itself. As seen with Ichigo Kurosaki, one must become one with their Bankai to finally release it. The shinigami must search deep in themselves and defeat the Bankai as a physical being to be able to use this power.
When Bankai is released the Shinigami usually yells or says "Ban-Kai" to release this power.
The form of the Bankai is not reserved to that only of a sword. The bankai takes its true form which can be anything from a katana to a giant weapon.
Bankai not only changes shape of the zanpakutou, but also gives the Shinigami Bankai specific abilities.
Know Bankai Users
Shigekuni Yamamoto Genryusai
Ginrei Kuchiki (assuming that he was a captain)
Rojuro ''Rose'' Otoribashi
Shihouin Yoruichi (assuming that she was a captain)