You must work towards the ending that implies you becoming a Blessed Messenger. To do this, collect all 49 red sigils, all 30 stars, and all 9 silver sigils. You must also discover the code to unlock the sixth level of the tower. The three numbers of the code can be found at the start of each Star World. They are written on the board directly in front of the teleporter. A Hub's Star World's board gives the first digit, B Hub's Star World the second, and C Hub's Star World the third. The code is randomly changed with each playthrough. After getting the complete code, use it to unlock the sixth level of the tower on one of the tower's terminals. Once there, solve the ultimate tetromino puzzle to become a Blessed Messenger. You will get the "Blessed Messenger" achievement at the end credits.