It's in the Sasuke v. Itachi arc.
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As for info, well bascially the user summons a monk like form that wields a sword and a shield. The sword of Tutsuka (which is what he wields) seals an opponents soul into another demension upon contact with said opponent. The shield is called Yata's Mirror and it's another legendary item which reflects any and all attacks made on the user. the combination of these two makes the user practically invincible. There are (however) many mystery's about this technique. Your supposed to unlock this technique after one mastering the other two MS techniques. However, the amount of chakra it takes to use it is unkown.