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 pays the cost of 100 piety, or 50 piety with Papal Investiture.
- The religious head dislikes the target and likes the requester. Vassal religious heads will also always grant the excommunication requests of their lieges.
- 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.
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.
- if vassal, 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:
- if using the (very expensive) "Make Declaration of Repentance" decision. You can ask him to lift the excommunication directly for 100 piety (50 with Papal investiture) if he likes you enough.
- if the character agrees, by event, to participate in a Pope's ongoing Crusade.
- if establishing an antipope, which ends excommunication as realm no longer obeys the Pope.
- upon religious conversion
Strategies
- 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)
- 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.
- 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.
Christians
- Send your Court Chaplain to Improve Religious Relations with your religious head.
- Go on a pilgrimage
Catholics
- Join a crusade
- Switch to Papal Investiture
- Give the Pope a landed title
- Control cardinals
Muslims
- Charity decision
- Go on hajj
- Join a jihad