Simeon told Mary that what would pierce her own soul?

Answer: A sword

Simeon said Jesus would be a sign spoken against, "and a sword will pierce your soul as well."

“Then Simeon blessed them and said to His mother Mary: “Behold, this Child is appointed to cause the rise and fall of many in Israel, and to be a sign that will be spoken against, so that the thoughts of many hearts will be revealed—and a sword will pierce your soul as well.””— Luke 2:34-35 (BSB)