Who was the Priest that was told that he would not die until he saw the Saviour?

The priest you are referring to is Simeon, a figure mentioned in the Gospel of Luke in the New Testament. According to the narrative, Simeon was promised by the Holy Spirit that he would not die before seeing the Messiah. When Mary and Joseph brought the infant Jesus to the temple for dedication, Simeon recognized Him as the Savior and joyfully proclaimed this fulfillment of the prophecy.