Some mercenaries and holy orders are vassals of feudal lords in historical starts, or can become vassals by special decision. Others can become vassals, but only under certain circumstances if they become landed.
Benefits of vassalizing companies
- A vassal company can only be raised by its liege, and can be hired regardless of the company's normal hiring limitations
- A vassal company charges only 1/5 the regular hiring cost. There is no upkeep cost.
- Holy orders of Pagan religions are allowed to raid, which greatly help their liege in upgrading their holdings. (Requires TOG.)
- With SoA:
- Many sons of rulers will become courtiers of the holy order. With the holy order under your control, you can land these characters to either press their claims or allow them to inherit their parent's title. (For the latter, you may need to reload or
recalc_succon their parent.) - You can remove members of other dynasties from the order's line of succession to help a knight of your dynasty become the next grandmaster. This further decreases the hiring cost, and gives monthly prestige and piety to all count+ rulers of your dynasty with the correct religion.
- Holy order captains can become incredibly rich from donations and effectively owning 10 castles. By appointing a wroth character as your marshal, you may get a chance to seize their wealth. Without Conclave, appointing a greedy character as your steward works too.
- You can retract the castles they erect, with minimal-to-no penalty to opinion from other vassals. You will inherit these castles, as their succession remains Appointment even after retraction. With fewer than 10 castles under their control, the holy order will be able to construct new castles for only 300g each, completing the cycle.
- Many sons of rulers will become courtiers of the holy order. With the holy order under your control, you can land these characters to either press their claims or allow them to inherit their parent's title. (For the latter, you may need to reload or
Methods
Offer vassalization
A landed mercenary company or holy order may, like any independent duke-tier ruler, accept diplomatic vassalization if the requester is a same-religion neighbor. As with any vassalization offer, you must be at least two of:
- Two tiers higher (emperor)
- Their de jure liege (you may grant one of your de jure duchies to a holy order while at peace, or use the technique described below to have a mercenary band inherit one of your de jure duchies)
- Same culture (note that each new captain will adopt the holy order's traditional culture)
Since rulers will only accept diplomatic vassalization if their would-be liege is significantly stronger than they are, it can be hard to vassalize a powerful holy order. One rather gamey way to appear more powerful than the order is to hire them and then deliberately deplete their troops, such as via futile assaults or having them stand around in high-attrition land. With the order temporarily weakened this way, the ruler will appear stronger.
Characters who are obedient toward you will also accept vassalization offers. It may be possible to inculcate obedience through 
the dark arts.
Claiming the holy order title
Alternatively, you may be able to find claimants. Open Elective succession does not generate claims for the second and third in line, but holy order captains may have children, who will inherit weak claims. See Activating weak claims for tips.
Force vassalization CB
Mercenaries, but not holy orders, can be targeted with the Force Vassalization CB. However, they must have land before you can declare war on them. Independent mercenaries can be given land in two ways:
- A) Become bankrupt while employing the mercenary band. They might invade a kingdom.
- B) Invite lowborn women and send them to marry heirs to mercenary companies. After a generation, some new mercenary captains will have sons. Invite a mercenary captain's son, give him land, then kill him.
Stealing vassal companies from other kings
Inheriting, claiming, or subjugating(?) a kingdom with a vassal company will give you control of the company.
Mercenaries:
- Papal Guards is a vassal of the Papacy whenever the Pope is landed. It can be retracted after vassalizing the papacy, usually by pressing an antipope's claim. The Pope must remain landed.
- Mamluks is a vassal of Egypt from 850
- Ghilman is a vassal of Persia from 1037, then Rum from 1157
- Varangian Guard is a vassal of Byzantine Empire from 950 until 1204, or when created by the Byzantine Emperor by decision in earlier starts
- Scholae Palatinae is a vassal of Byzantine Empire until 1082
Holy orders:
- Jerusalem may vassalize Knights Templar or Knights Hospitaller, and has both at later start dates
- Castille has the right culture to vassalize Knights of Calatrava or Knights of Santiago, and has both at later start dates
Vassalization by decision
- The King of Jerusalem may vassalize Knights Templar or Knights Hospitaller
- Scions of the Northern Crusade bloodline may vassalize the Teutonic Order for 300 piety (req +25 opinion; the Kingdom of Jerusalem must be held by a Catholic ruler)
Jomsvikings' special case
With conditions fulfilled, a event-spawned character named Panaltoke would call for the creation of Jomsborg, thus founding Jomsvikings. He shows up in the court of certain counties' holder. So for the player it is possible to grant Panaltoke a duchy when he asks, thus making would-be-founded Jomsvikings 'slip' (somewhat like inheriting) into the realm.
Keeping holy orders
However, to keep the holy order as a vassal, the grandmaster must have a reasonably good opinion of the liege (above -20) and be the same religion. Otherwise, the holy order may declare independence and walk away from the vassal contract.
Becoming the grandmaster
If the vassal company/holy order somehow changes its Succession law to a non Elective law (such as gaining a kingdom), the kingdom's succession law will supersede the "default" succession, forcing the Company/Holy Order to be passed along Dynastic routes. Since there is no Dynastic heir, you will be the Heir by default (inheritance by appointment), enabling you to possibly gain command of the company/Holy Order.
Take note, however: upon the Grandmaster's death, you inherit his wealth, the kingdom title you gave him, and any holdings he controlled directly. The vassalized Order will likely continue with a new elected Grandmaster as your vassal, presumably because it still has baronies that were indirectly controlled, i.e., held by other Knights o/t Order. It is uncertain how this may affect any holdings you may have granted with the Kingdom title. For safety, it is best to give an empty kingdom title, such as transferring the Duke of Brittany to another king, then granting the Kingdom of Brittany to the Order with no accompanying lands. If the Order has significant wealth, this may be worthwhile even if it only gains you wealth and a single barony.
It is possible to revoke the holy order title and become grandmaster yourself, though this seems to have little benefit over vassalizing them. The duchy title may deteriorate your vassal relations due to the -10 "Too many held duchies" penalty. You can raise them for free (no initial cost nor maintenance—the Military screen may show an "upkeep" but it does not actually deduct anything). You may be disallowed from arranging marriages while you hold the title.