Every AI clan of every kingdom periodically checks if it should leave its kingdom in search of better opportunities. There are two options they consider - just leaving and become independent for a time or defecting to another kingdom. These terms came from the game code and will be used a lot both on this page and in the mod itself, so, please, note them. Natively, leaving always means that clan will abandon any fiefs it has, while defecting means that clan may take their lands with them, if the kingdom they are defecting to (aka target kingdom) is at war with their current kingdom. Now this is how things work in vanilla. The problem with this logic, as I see it, is that evaluation process somewhat simplistic, which leads to a situation when clans leave their kingdoms seemingly out of the blue and maintaining a kingdom becomes frustrating for player.

Ensured loyalty system adds several adjustable conditions to skip above consideration logic partially or completely, depending on your tastes and desire for "fair and balanced" or just comfortable gameplay.

First of all, it adds oath of fealty limitation periods, so clans can't change factions the way one changes clothes. Same goes to mercenary contracts, but to a lesser extent. This skips considerations for clans completely, if they joined a kingdom not long ago. My intention with that is to add some stability to the game while not changing much.

Then comes "Achieve via relation" option. Roughly speaking, it forces clans to stay with their kingdoms, if relationship between clan leader and kingdom leader is high enough. There are lots of options that can be used to adjust required relation to your liking (taking into account faction type, leaders honor level, being landless etc.), but mostly they are self-explanatory, so I won't overexplain them here.

The one that requires some explanation, though, is "Withhold price". My thoughts behind this option is that for some situations, when clan really would like to leave, making them stay just because of strong friendship do seem like an overkill. To compensate for this, kingdom leader should pay some influence (and optionally even gold) to withhold clan in the kingdom.
In terms of game mechanics, when clan considers leaving or defecting, game calculates scores for these actions, which represent clan's willingness to leave (and in case of defecting - also the willingness of target kingdom to accept clan). Sum of these scores should be below some fixed threshold for clan to stay. If it is higher, clan would natively leave, but ensured loyalty system comes in play and forcing the clan to stay. With this option enabled, the amount, by which "stay" threshold exceeded, is taken into account, and the higher it is, the higher would be the price to withhold clan in the kingdom. Players will always have a choice if they are willing to pay this price or let the clan go, while AI faction leaders have all new logic to help them make such decision. Note that relation between clan and kingdom leaders still should be high enough, otherwise clan would just leave, as they do in vanilla.

Consider my Kingdom mod functionality is there. I have plans to expand it and make some other tweaks to clan migration between kingdoms, including adding a way of vassalization of minor factions.

Not yet described