... the Council of Trent was obliged to enact the severest penalties against it (dueling). It decreed that "the detestable custom of duelling which the Devil had originated, in order to bring about at the same time the ruin of the soul and the violent death of the body, shall be entirely uprooted from Christian soil" (Sess XXIV, De reform, c. xix). It pronounced the severest ecclesiastical penalties against those princes who should permit duelling between Christians in their territories. According to the council those who take part in a duel are ipso facto excommunicated, and if they are killed in the duel they are to be deprived of Christian burial. The seconds and all those who advised the duel or were present at it are also excommunicated. These ecclesiastical penalties were at a later date repeatedly renewed and even in parts made more severe. Benedict XIV decreed that duellists should also be denied burial by the Church even if they did not die on the duelling ground and had received absolution before death. All these penalties are substantially in force today. Pius IX in the "Constitutio Apostolicae Sedis" of 12 October, 1869, decreed the penalty of excommunication against "all who fight duels, or challenge to a duel or accept such challenge; as well as against all who are accessory to the or who in any way abet or encourage the same; and finally against those who are present at a duel as spectators [de industria spectantes], or those who permit the same, or do not prevent it, whatever their rank, even if they were kings or emperors".

https://www.newadvent.org/cathen/05184b.htm