Zechariah 12:10 says they will look on the One whom they have done what?

Answer: Pierced

God will pour out a spirit of grace and prayer, and "they will look on Me, the One they have pierced," mourning as for an only child.

“Then I will pour out on the house of David and on the people of Jerusalem a spirit of grace and prayer, and they will look on Me, the One they have pierced. They will mourn for Him as one mourns for an only child, and grieve bitterly for Him as one grieves for a firstborn son.”— Zechariah 12:10 (BSB)