- Tributary is the most independent type of subject. The subject pays 25% (35% if the overlord has the Protection Racket tradition) of its mineral and energy income to the overlord, but in turn the overlord is unable to attack it. Neither the overlord nor the Tributary have any obligation to help one another when attacked or attacking and the tributary is free to expand and conquer. If this state is acquired via warfare, a cost free Guarantee of Independence is signed with the Tributary. However this Guarantee can be revoked at any time with the usual penalties. Due to the Guarantee and Subject Relationship, Trust grows quickly up to 100. This state can be converted into a Protectorate or Vassalage per the usual rules, with the Trust acting as a big positive modifier.
- Protectorate is a subject protected by a technologically superior Empire. The protectorate gains a huge research-bonus (90%) to all technologies that their overlord has already researched and is automatically converted to a vassal once they researched 50% of the overlord's current tech. Any pre-FTL species that is technologically enlightened is automatically created as a Protectorate under whichever Empire granted them the ability to space-travel. The overlord in turn gains +0.25 influence each month. The overlord will defend the protectorate when attacked, but the Protectorate is not drawn into wars unless it turns into a vassal. In addition the protectorate cannot colonize beyond their borders or declare wars.
- Vassal is the most controlled type of subject-Empire. They will automatically join their overlord's wars, aggressive and defensive, and they have no autonomy when it comes to foreign policy or diplomatic relations, nor can they colonize new worlds that are outside their border range. If there are significant technological differences between both empires, this option is replaced by the Protectorate status. Planets owned by vassals count towards their overlord's victory conditions.
Subjects of Fallen Empires
Should a Fallen Empire awaken, they will offer every empire the option to become their subject, sparing an empire their potential wrath.
- A Satellite is a subject of the Watchful Regulators. They must pay 25% of their research production to the Fallen Empire and are banned from using artificial intelligence.
- A Thrall is a subject of the Jingoistic Reclaimers. They can not colonize new worlds and pay 25% of their Mineral and Energy income to the Fallen Empire. However they can declare war and conquer worlds from each other.
- A Dominion is a subject of the Doctrinal Enforcers. They must pay 25% of their Mineral and Energy income to the Fallen Empire and their ethos are changed to Fanatic Spiritualist and Authoritarian while their authority and civics are randomly switched to suit the new ethos. Hive Mind empires do not change ethos however.
- A Signatory is a subject of the Benevolent Interventionists. They have no autonomy over their diplomatic policy and are forbidden from purging or conducting slavery but can expand and the Benevolent Interventionists will come to their protection should they be attacked.
For subject empires the attitude is replaced by loyalty, expressing how content they are living under their Overlord’s rule. If a subject's loyalty becomes low they have a chance to start a war for independence, often waiting for an opportunity when they sense weakness (a taxing war, a larger uprising, a galactic crisis etc.). The loyalty is a compound of a few different parameters but the main ones are the subjects opinion of the Overlord, the total military strength of all the subjects relative to the Overlord as well as if the subject can find someone who supports their cause.
An Empire is able to support the independence of another Empire's vassal. Doing this will greatly increase the vassal's Liberty Desire (assuming the supporter has a fleet rivaling that of the vassal's Overlord) and may cause them to rebel. If they do, the Empire supporting them automatically joins their war for independence. An Empire is also able to guarantee the independence of a smaller Empire, automatically entering the war on their side should they be attacked.
Subject empires do not suffer from border friction.
Subjects that an empire possess for longer than 3600 days (10 years) can be peacefully "integrated" by paying influence, at a rate of 5 influence a month. The base cost is 20, increased by 10 for each pop and 50 for each planet. Only one vassal can be integrated at a time. Both the time before integration and the integration cost can be reduced through Domination traditions.
Once the integration is completed, all planets and ships of the vassal are transferred to the overlord. The ships and the spaceports are left with their graphic culture. All the vassal leaders are exiled and all their armies are erased, as well as all the resources they have. New scientists are required for their science ships.
Releasing a vassal
A player may release the home planet of an alien species as a vassal through the "demographics" page. If the player has conquered other planets belonging to this species, these may be given to the vassal after liberation. This may be a good idea for a Xenophobic Empire.