Vassalage

The feudal hierarchy — a liege draws power from its vassals, but only while its own members remain strong.

Vassalage is the liege/vassal half of Faction Relationship, and the only diplomatic state that changes a faction's numbers. It is what makes the game medieval rather than merely territorial.

The bargain

A liege adds a share of each vassal's Faction Power to its own, scaled by factions.vassalPowerContributionMultiplier. That contribution lifts the liege's Demesne Limit, letting it hold land its own membership could not support.

The vassal, in exchange, keeps its own identity, its own land, and its own roster — and typically gains the protection of the vassalageTreeCanInteractWithLand and liegeChainCanInteractWithLand Faction Flag settings, which let the hierarchy build inside each other's claims.

The condition that keeps it honest

Vassal power is added only when the liege's own member power is at least half of its maximum. Below that threshold the contribution is not reduced — it is removed entirely.

This single clause is what prevents the obvious exploit. Without it, a one-player faction could accumulate vassals and hold an enormous demesne on borrowed strength. With it, an empire is capped by the activity of its own core: let the capital go quiet and the whole structure snaps back to what its own members can carry.

A tree, not a list

MfVassalNode is recursive — a faction id plus a list of child nodes — so a faction can have vassals which themselves have vassals, and the whole subtree can be searched or flattened with contains, map and flatMap. Its counterpart MfLiegeNode points the other way, holding a single optional liege, because looking up the hierarchy is a chain rather than a tree.

Power contribution is computed from the direct vassals of a faction, and each of those factions has already folded in its own vassals' contribution — subject, at each level, to that level's own half-strength test.

Related

Becoming someone's vassal is a consequential, mutual act, which is why it is one of the three Approval Request types.