Sanctus claimed that Dante's blood was stronger then Nero's that's why he was trying so hard to use Dante but since he couldn't catch him he used Nero instead, this was only to power The Savior and fight with it. But he still tried to catch Dante even asking him what would alter his position. Also it says to complete The Savior he will need 3 things. A Sword of Sparda(that's just what it says) Sparda's blood and a priestess and they all are suppose to fuse or something to create enough power to complete it, just like how it was in DMC3 to power the Tower.This should be a whole new thread.