Robert the Pious, immediately after his excommunication
Excommunication is an act of censure by a religious head. The church officially cuts all ties with the character, causing major diplomatic issues. It is available for most branches of Christianity, and for the Yazidi heresy of Sunni Islam. With 
Holy Fury, reformed pagan faiths with the Hierocratic or Autocephalous leadership type also have this mechanic.
Requesting an Excommunication
Any ruler can ask for another ruler of the same religious denomination to be excommunicated if:
- The requester can pay the piety cost
- The target does not control the religious head. This typically means that the Byzantine Emperor can never be excommunicated.
- The target does not control more cardinals than the requester (for Catholics, with
Sons of Abraham DLC). - The target did not recently have an excommunication lifted.
- The moral authority is at least 40%.
A few event chains can also lead to excommunication.
Cost
The cost to request an excommunication is based on the rank of their highest-rank relative. In addition to their target, relevant relatives include parents, grandparents, siblings, and children.
| Highest rank relative | Cost | Cost with Papal Investiture |
|---|---|---|
| (Courtier) | 25 | 12.5 |
| Baron | 50 | 25 |
| Count | 75 | 37.5 |
| Duke | 100 | 50 |
| King | 125 | 62.5 |
| Emperor |
Effects
Being excommunicated will:
- harm opinion: -5 diplomacy and an additional -15 opinion from characters of the same religion. For rulers, this makes it considerably more difficult to manage a realm, and makes faction rebellion more likely.
- give every ruler in the same ecclesiastical branch an Excommunicated Ruler CB which they can use to force the ruler to abdicate.
- Liege may imprison excommunicated vassals without incurring tyranny, making this a potent tool to deal with a troublesome vassal. Excommunicated characters who are imprisoned can also be executed without penalty.
Excommunication will end:
- upon using the (very expensive) "Make Declaration of Repentance" decision and agreeing to the Pope's demands
- upon using the "Lift Excommunication" interaction for another character
- if the character agrees, by event, to participate in a Pope's ongoing Crusade.
- when establishing an antipope, which ends excommunication as realm no longer obeys the Pope.
- when an antipapacy ends, for characters following the antipope
- upon religious conversion
Strategies
Avoiding excommunication yourself
- Keep your religious head's opinion of you positive, especially if you know another ruler would like to see you excommunicated
- Bribe your religious head. If Catholic, bribe a newly elected Pope, before gifts become too expensive.
- Send your Chancellor to Improve Diplomatic Relations (less effective in Constantinople, where you have a 50% chance of pleasing the Basileus instead)
- If possible, vassalize your religious head. You'll be protected from hostile actions (including excommunication); if you choose to, you may (ab)use the powers of your religious head to cement your dynasty's grip on power.
Damaging opponents
- Excommunicate vassals for free imprisonment and/or execution
- Excommunicate rivals to soften up their realms before attacking.
- Depose excommunicated rulers to make their child heir inherit. This makes the realm vulnerable to weak claims.
- Reinstate deposed rulers - newly abdicated rulers gain a strong claim on their old realm. If you grant them a landed title, you can push their claim and thus expand.
- Excommunicate your heir for free imprisonment, and subsequently order them to take the vows to get rotten heirs out of the way of your strong genius younger son.
Exclusive to Christians
- Send your Court Chaplain to Improve Religious Relations with your religious head.
- Go on a pilgrimage
Exclusive to Catholics
- Join a crusade. Beware: if you pledged to join a crusade during the preparation phase, but did not send troops, you will eventually be excommunicated (via event); you'll first be given a warning event which causes you to lose piety.
- Switch to Papal Investiture. With Holy Fury, emperors who have vassalize the pope should have Free Investiture before their (and their successors') coronation by the Pope, so that the Pope can ask for a law change rather than independence (which is another valid and popular request)
- Give the Pope a landed title
- Control cardinals
- If you intend to take the Papal States, consider setting up an antipope, as you are highly likely to be excommunicated and and unlikely to have the excommunication lifted.
Exclusive to Muslims
- Charity decision
- Go on hajj
- Join a jihad