An antipope is a rival claimant to the Papacy and to the bishopric of Rome.
Their title is simply "Pope" (not only because they claim to be the true pope; historically, it can be hard to distinguish between legitimate popes and antipopes). An antipope has two of the powers that popes have: excommunication and taxation.
Creation
Secular rulers can appoint a vassal bishop as an antipope, if:
- the ruler is independent and at least duke tier.
- the ruler can spend 500 Prestige
- the bishop is at least count tier
- the bishop is not subject to Papal Investiture crown law
- the bishop likes the ruler more than the pope
- the true pope is not the ruler's vassal
- the religion does not have the "Deposed an Antipope" modifier to moral authority (Modifier is applied upon successful Depose Antipope war and lasts 50 years)
To appoint a bishop as antipope, go to the Religion tab (F9) and click the hat button next to the bishop.
Benefits
There are several benefits for the ruler:
- Every bishop in the realm of the antipope's controller will get the "Head of Religion" opinion bonus of +30 towards the antipope rather than the real pope.
- Every bishop that prefers the antipope over the proper pope and their liege will pay their taxes to the antipope.
- Antipope controller can request excommunication of anyone in their realm
- The pope cannot excommunicate the controller of the antipope or his/her vassals (conversely, the antipope cannot excommunicate those loyal to the pope, or to another antipope).
- Allows emperors to vassalise the Papacy, by pressing the antipope's claim. Once successful, the emperor will be protected from hostile papal actions, as a vassal pope cannot carry out such actions against his liege.
- As it is often easier to please an antipope,
coronation requests from antipopes are usually more acceptable than those from the Roman pope.
Risks
- Rulers who prefer the pope in Rome have a Depose Antipope casus belli against any controller of an antipope.
- The Roman pope often joins any "depose antipope" wars on the side of the attacker.
- The Roman pope may ask other rulers to declare Depose Antipope wars on you (as a coronation request)
- A vassal can form an Antiking faction to install a claimant to your primary title, institute Papal Investiture, and depose the antipope.
- Each existing antipope reduces the Moral authority of the Catholic church by 30%; the penalty on moral authority may favor the spread of heresies and make it harder to convert provinces.
- Once any antipope is deposed, another antipope cannot be established for 50 years.
Pushing an antipope's claim
The appointer can press the antipope's claim upon the Papacy through war. Since Patch 2.0, rulers loyal to the current Pope may join the war.
If successful:
- the antipope becomes the new head of the Catholic church. The ex-pope will become an antipope with no successor, and the split in the church will be mended after his death.
- new pope gets a +100 relations modifier towards the former appointer.
- should the ruler pressing the claim be an emperor, the pope will become a king-tier vassal.
- Vassal popes collect taxes from many Catholic bishops, passing along a portion depending on how you tax vassal theocracies. However, a vassal pope will stop paying taxes entirely if his opinion drops below 0.
- Vassal popes are likely to accept favor requests (excommunication, claims, ...).
Technically, being an antipope is equivalent to having a claim on the papacy. Bugs have been known in which children of popes have inherited claims and therefore appeared as antipopes.
Using your antipope
Your pope will accept most papal requests from you, leaving you with only the piety cost and the relations penalty. You can now request claims on counties and duchies, even outside your realm if you successfully press your pope's claim. You can also excommunicate someone you want to imprison, plot against, or steal courtiers from (using "invite to court").
An antipope can be a major asset if the current pope doesn't like you, and you have political enemies with high enough piety, and good enough graces with the pope, to get you 
.
Note that while a vassal pope will always agree to your requests, they will still charge double piety and request investiture law change if you're using free investiture, and incur the normal opinion penalties if you overuse his services. Also, at negative opinion, a vassal pope will simply not pay taxes instead of a gradual reduction. (Granted, this is still a tolerable scenario, considering the fact that wealth is not leaving the realm, but remaining in the pope's hands to be used at his discretion.) Remember that even if you do not use a vassal pope's services, you'll be protected from hostile actions (e.g. excommunication) simply by being the pope's liege. With Holy Fury, a vassalized Pope may ask for independence as a request for a coronation. To counter this, set the realm's Investiture Laws to "Free" so that the Pope can request for a law change rather than independence.
Maximizing taxation
To collect lots of taxes, even from outside of your realm, pick a pope who will be loved by bishops:
- High piety (+1 opinion per 25 piety, up to +20 opinion at 500 piety)
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- High church opinion
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- Positive "same trait" opinion
- Most of the above, plus
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- Most of the above, plus
- High diplomacy
- Lowborn, for a +5 "same dynasty" opinion from most bishops
You can increase the piety of your pope by granting him multiple bishoprics, making him a flank leader in battles against religious enemies, and appointing him as court chaplain.
If you give your pope prince-bishopric or higher, he can:
- Have bishops as vassals, for additional piety
- Have a chancellor, for improved state diplomacy
- You can help him get a good one by matrilineally marrying a potential chancellor into his court, or giving him one as a vassal.
- Choose the "Theology" focus, for +20 church opinion
- Most likely to choose Theology if: ,
- More likely to choose (Theology or Scholarship) if: theology education (any level),
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- Most likely to choose Theology if:
- Benefit from "Dynasty controls Catholic" and "Dynasty controls <holy order>" (+1 piety and +2 prestige per month, each)
You can also interfere with the effectiveness of your rival in the Vatican:
- Murder an effective pope.
- Murder an effective cardinal, especially a preferatus.
- Fill the College of Cardinals with men who would be ineffective popes. Note that bishops in your realm cannot become cardinals once you have an antipope.