The ordination of priests - co-workers of the bishops
"Because it is joined with the episcopal order the office of priests shares in the authority by which Christ himself builds up and sanctifies and rules his Body. Hence the priesthood of priests, while presupposing the sacraments of initiation, is nevertheless conferred by its own particular sacrament. Through that sacrament priests by the anointing of the Holy Spirit are signed with a special character and so are configured to Christ the priest in such a way that they are able to act in the person of Christ the head."
Source: Catechism of the Catholic Church (Vatican.va)