This is not intended as a historical form of government, but my usual gaming group is very cooperative, and I imagine this is the sort of arrangement a close-knit adventuring party might come up with to share rulership between them:
Founding an Oligarchy
An oligarchy can be formed either by settling unclaimed land or by conquering an existing domain.
Leadership and Structure
An oligarchy is ruled by a Council of Lords, each of whom rules a canton within the domain. These cantons raise their own income (earned by their Councillor) and garrison their own lands, but if a Councillor wishes to do any of the following, they must gain the approval of the Council:
- Invade another domain.
- Appoint a grand general for the combined armies of the domain.
- Demand a duty from any vassal of the domain.
- Appointing a new vassal to the domain.
The Council may also be granted other powers, but these are the minimum required for an oligarchy.
Religion
When an oligarchy is established, it must be established as either Lawful of Chaotic. All Councillors must either share this alignment or be Neutral and be prepared to abide by the strictures of the domain’s alignment.
An oligarchy may have a unified religion with a single spiritual advisor for the whole domain, or alternatively it may allow each canton to have it own religion and spiritual advisor.
Cantons
When an oligarchy is established, its lands must be divided into Cantons, each Canton being ruled by a Councillor. The Councillor who rules a canton receives the land, service and tax income from families living in the canton and pays garrison, liturgy and tithes as normal. Each canton pays a special tribute (typically 3gp per family) to the Council, which then disburses the revenue it receives from this tax and from tribute equally to all councillors.
Cantons have their own morale, which is based on their ruler’s Charisma as normal. If a councillor has insufficient level to properly hold their canton, the canton will suffer a morale penalty as normal.
Securing the Realm
The total value of all strongholds in the realm needs to be sufficient to control the entire domain. If not, all cantons within the domain will suffer a morale penalty. It doesn’t matter how these strongholds are distributed within the domain.
Vassals
An oligarchic domain may have vassals like any other domain. Vassals swear fealty and pay tribute to the Council, not to any Councillor. So long as a vassal is personally loyal to a Councillor they are treated as a henchman vassal (their morale is based on the CHA of the councillor they are loyal to), otherwise they have a base morale of -2 as normal.
Devolution
If a Councillor wishes to secede from the domain, they require a unanimous vote from the Council. Without this they must attempt to secede by force – attacking their former allies until they concede to allow secession. While the secession war is ongoing the rebellious canton suffers -2 morale.