The Tomb of the Holy Innocents is under the Basilica of the Nativity of Christ in Bethlehem. The relic is a piece of their tomb. They were martyred in the 1st Century and their feast day is December 28. 

The Holy Innocents are the male infants of the town of Bethlehem, the birthplace of Jesus, whose deaths were recorded in Matthew 2:1-18. The infants were put to death by the soldiers of King Herod the Great (r. 37-4 B.C.) in order to prevent the fulfillment of the ancient prophecy that the King of the Jews would be born in the little town. The exact number of the slain is unknown, but it can be deduced that the population of Bethlehem during that period was not large. An angel warned Joseph of the impending danger and thus the Holy Family escaped into Egypt. The fourth-century poet Prudentius called the infants the flores martyrum, “flowers of martyrdom.” According to legend, one of Herod’s own sons, who was with a wet nurse in Bethlehem, was also slain. 

MATTHEW 2:1-18

When Jesus was born in Bethlehem of Judea, in the days of King Herod, behold, magi from the east arrived in Jerusalem, saying, “Where is the newborn king of the Jews? We saw his star at its rising and have come to do him homage.” When King Herod heard this, he was greatly troubled, and all Jerusalem with him. Assembling all the chief priests and the scribes of the people, he inquired of them where the Messiah was to be born. They said to him, “In Bethlehem of Judea, for thus it has been written through the prophet: ‘And you, Bethlehem, land of Judah, are by no means least among the rulers of Judah; since from you shall come a ruler, who is to shepherd my people Israel.’”

Then Herod called the magi secretly and ascertained from them the time of the star’s appearance. He sent them to Bethlehem and said, “Go and search diligently for the child. When you have found him, bring me word, that I too may go and do him homage.” After their audience with the king they set out. And behold, the star that they had seen at its rising preceded them, until it came and stopped over the place where the child was. They were overjoyed at seeing the star, and on entering the house they saw the child with Mary his mother. They prostrated themselves and did him homage. Then they opened their treasures and offered him gifts of gold, frankincense, and myrrh. And having been warned in a dream not to return to Herod, they departed for their country by another way.

When they had departed, behold, the angel of the Lord appeared to Joseph in a dream and said, “Rise, take the child and his mother, flee to Egypt, and stay there until I tell you. Herod is going to search for the child to destroy him.” Joseph rose and took the child and his mother by night and departed for Egypt. He stayed there until the death of Herod, that what the Lord had said through the prophet might be fulfilled, “Out of Egypt I called my son.”

When Herod realized that he had been deceived by the magi, he became furious. He ordered the massacre of all the boys in Bethlehem and its vicinity two years old and under, in accordance with the time he had ascertained from the magi. Then was fulfilled what had been said through Jeremiah the prophet: “A voice was heard in Ramah, sobbing and loud lamentation; Rachel weeping for her children, and she would not be consoled, since they were no more.” 

(Bunson, Matthew E. Our Sunday Visitor’s Encyclopedia of Saints. 2nd ed., Our Sunday Visitor Inc.,u.s., 2014.)