Religion

Religion
Has a more direct impact on diplomacy. In the ancient world priests were often arbitrators of diplomacy, which means all negotiations that occur between two states that share a religion may have a diplomat to settle their dispute. If a treaty is resolved with a mediator overseeing it that means that the treaty is also made to the third party. So any oath breaking done is done not just to the secondary kingdom but also to the diplomat. Eg France and spain make a treaty with a catholic priest overseeing it. If Spain breaks the treaty, Spain is now an oath breaker not just to France but to all of Catholicism. The Catholic church may absolve Spain which removes their Oath breaking to the Catholics but there will still be an oath broken to France. Now lets imagine that before Spain breaks the Oath, France is excommunicated by the Catholic church. If you are excommunicated that means that all parties that break an oath with you are automatically absolved. 

Excommunication
Being absolved is entirely at the discretion of the pope. I would like the pope to be an AI but this may prove  difficult. One option is that after a player is killed they get to become the pope.

Any oath you break with an excommunicated party is automatically absolved by the pope. You will still have an oath break with the excommunicated party.