I'm going to guess you're looking for this ancillary:
Code:
{Roman_Praetorian_Legion} Praetorian Guard
{Roman_Praetorian_Legion_desc}
This man is the Praetorian Prefect (Praefectus Praetorio), the current commander of the Praetorian Guard. The holder of the post was chosen by the emperor from among the equestrians. The special position of the praetorians made them a power in their own right in the Roman state, and their prefect, praefectus praetorio, soon became one of the most powerful men in this society. The emperors tried to flatter and control the praetorians, but they staged many coups d'état and contributed to a rapid rate of turnover in the imperial succession. The praetorians thus came to destabilize the Roman state, contrary to their purpose. The praetorian prefect became a major administrative figure in the later empire, when the post combined in one individual the duties of an imperial chief of staff with direct command over the guard also. Diocletian greatly reduced the power of these prefects as part of his sweeping reform of the empire's administrative and military structures.
{Roman_Praetorian_Legion_effects_desc}
Command +1 TroopMorale +1 Influence +1
export_descr_ancillaries entry, first part:
Code:
Ancillary Roman_Praetorian_Legion
Image PRAETORIAN.tga
ExcludeCultures barbarian, carthaginian, eastern, egyptian, greek
Description Roman_Praetorian_Legion_desc
EffectsDescription Roman_Praetorian_Legion_effects_desc
Effect Command 1
Effect TroopMorale 1
Effect Influence 1
Effect Subterfuge 1
Effect Negotiation 1
Triggers:
export_descr_ancillaries entry, second part:
Code:
Trigger trigger_PRAETORIAN
WhenToTest CharacterTurnEnd
Condition EndedInSettlement
and FactionType romans_brutii
and IsGeneral
and SettlementName Roma
and SettlementBuildingExists >= praetorian_training
and not FactionwideAncillaryExists Roman_Praetorian_Legion
and Attribute Subterfuge < 1
and not Trait City_Has_Governor > 0
AcquireAncillary Roman_Praetorian_Legion chance 100
So in general: be a general, in Roma, with the praetorian_training building built. Don't be a governor (it's ok, if you just move someone in for a turn they won't get that trait)
The critical thing is this: "Attribute Subterfuge < 1"
Yes, generals can get this, it's used to control their gaining of ancillaries, very clever
What ancillaries give this? Ones like:
"Roman_XXVIII_Legion"
Code:
{Roman_XXVIII_Legion} Legio XXII Primigenia
{Roman_XXVIII_Legion_desc}
This man is the current commander of Legio XXII Primigenia.
Easiest way to get your desired Praetorian guard is probably to just move a general (that doesn't have the governor trait) with no ancillaries to Roma to pick it up.
FYI, the required building is just what it sounds like from the internal name:
Code:
{praetorian_training} Praetorian Guard Training Grounds
You may or may not be able to stack a bunch of different "legion command" ancillaries on a single general... Not that these do much other than give +1 morale, which you should hardly need anyway.