Cardinal Robert Sarah has become the “proto-deacon”: that is to say that in case of conclave, he will be responsible for announcing the name of the future new pope, and will thus pronounce the famous “Habemus papam” from the balcony of St. Peter’s Basilica, noted the newspaper La Croix on May 21, 2018.
Cardinal Sarah, 72, has become the eldest of the cardinals-deacon voters, following the consistory of May 19. The older cardinal-deacon, the Italian Raffaele Martino, is no longer a voter, being more than 80 years old. On May 19, Pope Francis made changes in the order of the cardinals, some passing from deacons to priests.
In the past, the cardinal proto-deacon was also the one who crowned the pope with the tiara—symbol of the pope’s triple power as bishop of Rome, head of state of the Vatican, and universal teacher. Since the abolition of the coronation after Paul VI, the cardinal proto-deacon places the pallium, symbol of pastoral authority, on the shoulders of the pope at the inaugural mass of the pontificate.
The title of Protodeacon is more prevalent in the Eastern churches, but Wikipedia explains more:
In the Roman rite, the senior Cardinal Deacon is the Cardinal Protodeacon of the Holy Roman Church. He has the privilege of announcing the new Pope‘s election and name (once he has been ordained to the Episcopate) in the Habemus Papam announcement given from the central balcony at St. Peter’s Basilica in Vatican City State. In the past, during papal coronations, the Proto-Deacon also had the honor of bestowing the pallium on the new pope and crowning him with the papal tiara. However, in 1978 Pope John Paul I chose not to be crowned and opted for a simpler papal inauguration ceremony, and his three successors followed that example. As a result, the Cardinal protodeacon’s privilege of crowning a new pope has effectively ceased. However, the Proto-Deacon still has the privilege of bestowing the pallium on a new pope at his papal inauguration. Acting in the place of the Roman Pontiff, he also confers the pallium upon metropolitan bishops or gives the pallium to their proxies.