Sphere of Influence
A Sphere of Influence is the diplomatic influence held by one nation over its neighbours.
In Divine Wind, every nation in the player's sphere will grant the player additional magistrates as well as a boost in the player's diplomatic skill. The cost, however, will also rise as more and more nations come under the player's sphere. The range of the player's potential sphere is now also limited by trade range. It costs now 2+2n Prestige to include a nation in the player's sphere, where n = number of countries already in your sphere of influence. Also, you cannot make some diplomatic actions toward a nation that is in a third country's sphere of influence if you have truce with this third country. A large sphere of influence can be used to counteract the negatives associated with having high infamy or a poor Diplomacy ruler. A nation can leave a sphere of influence if they increase in power militarily or economically. If the large nation declines to defend a nation in its sphere, it can also lose influence. In addition a nation can in certain cases expand its sphere over a nation which is already in another nation's sphere, this can be useful for declaring war without worrying about interference from other powerful nations.
Heir to the Throne
A nation needs to have less income and less military power in order to come under another nation's Sphere of Influence (SoI for short). It costs 10 Prestige to include a nation in the player's sphere. If any nation performs an action with a nation within the player's sphere, the player will receive a Violated Sphere of Influence Casus Belli, this includes declarations of war and a SoI supersedes guarantees.
The possible range of the sphere is determined by prestige. If the player's prestige drops too low, nations may break free from the sphere. Declining a request for help in a defensive war from a member of the player's sphere will not cause a prestige hit unlike a guarantee.