Hordes are perhaps the most dangerous enemy in Crusader Kings II. They have access to the Tribal Invasion casus belli and have or gain event troops. The leader of a horde always appears before the army and starts unlanded, unless the horde already exists when you start the game.
The Hordes
Magyars
The Magyars are a bit different from other hordes in that they start out in 867 with a substantial amount of land east of the Carpathians, and both King Álmos Árpád and his approximately 8,000 strong event-spawned army are already present at the Old Gods bookmark. He starts off with a tribal invasion of Hungary against the King of Bulgaria. Although Bulgaria is Orthodox, the counties comprising the de jure Kingdom of Hungary are still mostly Avar Tengri, with some Catholic Bohemian and Croatian territories.
Once Árpád or his heir has ended their initial war and taken at least everything in the Duchy of Pest and one of the Duchies of Transylvania, Ungvar or Nyitra, they may decide to stop being nomads and settle down. This does the following:
- Everyone gets an event announcing the formation of Hungary.
- The King of Magyars:
- Becomes the King of Hungary. Since the Kingdom of Magyars is tribal, it gets destroyed. However, its laws and history are copied over.
- Gets the nickname "the Great" (unless he has a nickname already), 1000 gold and 1000 prestige.
- Moves his capital to Pest, usurping it if necessary. You should probably grab the Duchy of Pest for yourself before pressing the button.
- Loses all his claims. Press any you care about before pressing the button.
- Three stacks of troops spawn in Pest, the number of which seems to be twice the size of the king's maximum levies. If you wait to conquer the entire de jure Kingdom of Hungary before using this decision, this gives you a huge number of troops. They have 60% of normal attrition.
- All provinces in the realm in the de jure Kingdom of Hungary convert to the king's religion and Hungarian culture.
- In the parts of the realm in the de jure kingdoms of Taurica, Ruthenia, Cumania and Alania:
- 6 random provinces change culture and religion to that of a random non-Hungarian neighbour.
- Any province not owned by a player or the king is usurped by a randomly generated 25 year old male peasant leader of the province's culture and religion. They are easy pickings for anyone who cares to go conquering them.
The Magyars are generally less aggressive than other hordes. The AI forms Hungary as soon as possible, so their event armies are not huge and their expansion is not incredibly fast. Because of the huge number of troops, the Magyars are a common choice for a world conquest.
The Seljuk Turks
One of three "adventurer" style hordes, they are unlike other hordes in many ways. First, instead of appearing off the edge of the map with unstoppable death stacks, they instead appear in someone's court, gain prominence, and then go on an adventure.
In the year 965 or soon after, Seljuk will appear in a the court of a landed duke+ ruler whose culture is Turkish, Cuman, Uyghur, or Karluk. (If no such rulers exist, Seljuk cannot spawn.) If the game rule "Turkic Conquerors" is set to "Random", Seljuk may appear as soon as 809.
The newly created Seljuk is 18, has Turkish culture and the same religion as his liege, and is the founder of his eponymous dynasty. About 4 years later, he will get an event to begin his adventure. He will decide among three counties near the border of Khiva and Persia – Tus (in Khorsan), Merv, or Dihistan – whichever is not controlled by his top liege. (Seljuk will not get this event if he is 
, 
, a 
, or a king. This event is limited to Seljuk himself; if Seljuk is dead, his descendants are not eligible.)
Seljuk's adventure will then start. He will gain 500 gold and a ducal-level temporary title (if unlanded). He will gain a large amount of event troops: about 2x his target, with a minimum of 3,000 and a maximum of 60,000. He will declare war on the top liege of the decided target county, using the adventurer CB, even if landed. The event troops will spawn in his liege's capital if Seljuk isn't landed, and in his own capital if he is landed. His old liege will be notified of this. As he did historically, he will convert to Sunni when setting out on his conquests (if he isn't already Muslim).
Upon victory, Seljuk will receive the conquered duchy. He will lose 25 decadence (if he's still Muslim), gain 1000 prestige, and spawn 6 stacks of 5098 troops of the exact composition of the Mongol's troop stacks. Unless he has a player as a liege, he will become independent and gain a titular empire called "Seljuk Empire".
Seljuk and his descendants will then usually conquer as much land as possible, often using the special Manifest Destiny Invasion CB.
Additionally, between 980 and 1100, one of Seljuk's successors may receive reinforcement troops, with a higher chance of happening if he/she has high Martial. If the event fires, he/she will lose 75 decadence and receive 4 stacks of 5098 of the same composition as before. He/She will also receive up to 3 additional stacks if he/she has high Martial. He/She will receive 1 additional stack with a Martial of 9, 2 with 11, and 3 with 13.
Strategies:
- Starting on 1 January 965, frequently search for "Seljuk" in the character finder. By noticing exactly when Seljuk spawns, it is possible to:
- Predict his attack date
- Invite him before he gains a council position, so he doesn't attack you (or even expands your realm for you). He's a Zealous Sunni, so you will need high State Diplomacy to bribe him enough to buy a favor.
- If you control Tus, Merv, or Dihistan, it may be wise to grant those counties independence. Then Seljuk won't target you initially, and more importantly, he won't get as many troops. Alternatively, you can induce these counties to rebel and be ready to enforce demands against them when Seljuk begins his attack.
Between the years of 965 and 1010, a brilliant strategist named Sabuktigin may appear. He will only appear in a realm that controls part of Kabulistan (specifically Ghazna, Bost, or Mandesh), in a Muslim court with duke rank or higher. If the game rule "Turkic Conquerors" is set to "Random", Sabuktigin may appear as soon as 809.
Nine years later, he will attack at one of the following four locations, whichever is not part of the same realm: Nandana in Gandhara, Multan, Lahore, or Bhakkar.
Rise of the Sultanate of Rum
Between the years of 1000 and 1200, an existing character may set out to found the Sultanate of Rum through a three-step invasion. The character must be an Altaic Muslim man, and his liege must also be Altaic and hold court in the de jure kingdoms of Mesopotamia, Persia, Syria, or Armenia. Characters from the Seljuk dynasty and demon children are especially likely to get this event.
The adventurer first targets a non-Muslim realm for a duchy in the Kingdom of Armenia or the Duchy of Azerbaijan. After 2 years warning, the adventurer gets 1.5 match_mult troops and declares war for the duchy. If successful, he will invade the Kingdom of Armenia with additional event troops, followed by invading Byzantine Empire for the Kingdom of Anatolia.
If the aspiring adventurer is thwarted from even declaring his first war, another may take his place after at least 10 years have passed. However, once the first war is launched, only this specific character can launch the subsequent invasions, and only while independent and at peace. By unlanding him or keeping him locked in war, it is possible to thwart his larger ambitions.
The Mongol Empire
The Mongol Empire, led by Genghis Khan Temujin, starts with all but one province of the kingdoms of Mongolia and Khotan, as well as the duchies of Zhetysu and Samarkand. They start with a Mongol Invasion of the Sultanate of Persia, and about 51,000 event troops in the 1220 "The Mongols" start. They, like the Magyars, are playable in ironman mode without cheats. Their initial holdings are nearly all nomadic, with the exception of the feudal duke of Karashar in the de jure kingdom of Khotan.
Temujin initially appears as the new ruler of the Khaganate of Khangai in 1196 and as the ruler of the newly created Mongol Empire in 1204, covering the kingdom of Mongolia and part of Khotan.
A number of holdings trigger a special event when sacked by the Mongols:
Eastern Europe:
- Barony of Kiev (County Capital), County of Kiev, Grand Principality of Kiev
- Barony of Smolensk (County Capital), County of Smolensk, Grand Principality of Smolensk
- City of Novgorod (County Capital), Grand City of Novgorod, Most Serene Republic of Novgorod
- Barony of Krakow (County Capital), County of Krakow, Kingdom of Poland
- Bishopric of Esztergom, County of Esztergom, Duchy of Esztergom, Kingdom of Hungary
South-East Europe:
- Barony of Constantinople (County Capital), County of Constantinople, Latin Empire
- Barony of Athens (County Capital), County of Atheniai, Duchy of Hellas
- Barony of Belgrade (County Capital), County of Belgrade, Kingdom of Hungary
Western Europe:
- City of Wien, County of Wien, Duchy of Austria, Holy Roman Empire
- Barony of Prag (County Capital), County of Prag, Duchy of Bohemia, Kingdom of Bohemia, Holy Roman Empire
- City of Aachen, County of Jülich, Prince-Archbishopric of Köln, Holy Roman Empire
- City of Paris, County of Paris, Kingdom of France
- City of London, County of Middlesex, Kingdom of England
- Barony of Lisboa (County Capital), County of Lisboa, Kingdom of Portugal
Southern Europe:
- Bishopric of Roma (County Capital), Prince-Bishopric of Roma, The Papacy
- City of Venezia (County Capital), Grand City of Venezia, Most Serene Republic of Venice
Near East:
- Mosque of Bagdad, Sheikhdom of Baghdad, Emirate of Baghdad (Abbasid)
- Mosque of Damascus, Sheikhdom of Damashq, Emirate of Damashq, Sultanate of Egypt (Ayyubid)
- Mosque of Jerusalem, Sheikhdom of Jerusalem, Emirate of Jerusalem, Sultanate of Egypt (Ayyubid)
- Mosque of Mecca, Sheikhdom of Mecca, Emirate of Hijaz, Sultanate of Egypt (Ayyubid)
North Africa:
- Mosque of Alexandria, Sheikhdom of Alexandria, Emirate of Alexandria, Sultanate of Egypt (Ayyubid)
- Wilayah of Cairo (County Capital), Sheikhdom of Cairo, Sultanate of Egypt (Ayyubid)
The Aztec Empire
Only appears with the 
Sunset Invasion DLC active, and there is additionally a rule to turn them off. Between 1250 and 1350 and with increasing likelihood past 1320, a randomly generated Nahuatl/Aztec character appears in one of ten Atlantic provinces with the Aztec Empire title (only the province's owner sees this event). Within 7 days rumors start to spread of people from across the Atlantic attempting to buy land (everyone sees this event).
At least two years later, 12 to 21 regiments of 5,400 troops (light infantry, heavy infantry and archers) spawn in this province and declare a tribal invasion on its liege for the de jure kingdom containing it. If they land in Iceland or the British Isles, they also get ships. Another similarly sized wave spawns in another Atlantic province that it doesn't already own an average of two years later and declares another tribal invasion there. All of these event troops, if they've survived, disband about 50 years after the Aztecs first arrive.
Timurids
Timur Timurid arrives as a courtier in any duke or higher Mongol realm in the de jure Empire of Persia between 1350-1370. If the game rule "Turkic Conquerors" is set to "Random", Timur may appear between as soon as 1200. Following the same pattern as Seljuk did, he will start his adventure after 9 years, using an adventurer CB against the owner of (in descending order and part of a different realm): Merv, Khorasan, or an equal chance of one of three provinces (Dihistan, Samarkand, Balkh). He starts with roughly two times his target's troops, although no lower than 3000 and no higher than 60,000. Upon success, he creates the Empire-level titular title of the Timurids and will gain new event troops totaling about 40,000, with a 50% chance each of six more armies of 5,098.
From the year 1370, any king or higher independent leader of the Timurid dynasty has a chance at triggering a "revitalization" event, decreasing decadence by 75, and spawning 35,000 new troops, with an addition 10,000 with each level of martial skill at 9, 11, and 13.
Strategies
- Don't enable Sunset Invasion (or alternatively, turn the rule off) if Aztecs would wipe you out (e.g., starting in Ireland in the year 1250)
- Expand quickly so you're powerful enough to hold them off, or at least have holdings in multiple de jure kingdoms so you won't be wiped in one war.
- Know when and where the Hordes will arrive
- For "off-map hordes", you will be notified several years before the armies arrive
- For "courtier hordes", periodically search for the created characters, starting in their earliest creation year.
- Assassinate horde leaders before they launch their attacks or gain heirs
- If you can't beat them, join them
- Convert to their religion, ideally before they gain the ability to revoke your titles through Medium Tribal Organization or Medium Crown Authority.
- If converting to Muslim, you must have the
Sword of Islam DLC: without it, becoming Muslim will cause game over!
- If converting to Muslim, you must have the
- Convert to their culture, and use Tribal Invasion yourself!
- Convert to their religion, ideally before they gain the ability to revoke your titles through Medium Tribal Organization or Medium Crown Authority.
- Send missionaries to convert the Mongol hordes to your religion
- Especially if your religion is Christian, so they lose their dreaded "Tribal Invasion" CB (inaccurate for last versions!)
- Marry for a non-aggression pact or even an alliance
- Grind them down
- Most hordes get a limited number of reinforcement event troops
- If you're lucky, the Golden Horde and Ilkhanate will fight each other
- Wait them out
- After 40 years, the first two Mongol hordes have a high chance of converting to a province religion
- After the year 1350, the Mongols and Aztecs are susceptible to the usual internal factions
- This makes 1350 a good time to assassinate Horde leaders (if the Hashshashin don't beat you to it)
- After the year 1350, organized religions can send missionaries to Aztecs