The Holy Spirit makes present the mystery of Christ
Together with the anamnesis, the epiclesis is at the heart of each sacramental celebration, most especially of the Eucharist:
You ask how the bread becomes the Body of Christ, and the wine . . . The Blood of Christ I shall tell you: the Holy Spirit comes upon them and accomplishes what surpasses every word and thought . . . Let it be enough for you to understand that it is by the Holy Spirit, just as it was of the Holy Virgin and by the Holy Spirit that the Lord, through and in himself, took flesh.
Source: Catechism of the Catholic Church (Vatican.va)