A Royal Marriage is a basic diplomatic option. Two monarchies of the same religious group can bind their dynasties together by arranging a marriage of dependents of the crowns. Non-monarchies are not eligible. Royal marriages last only until the death of a monarch.
A successful royal marriage will cause a one-time boost in relations. Because royal marriages can be entered and re-entered every time a monarch dies, they can be used to raise relations steadily over many generations. Two countries bound by a royal marriage will also see their relations improve (or improve faster, or just degenerate more slowly) every year as long as the marriage persists.
Royal marriages may result in inheritance, a Personal Union or a War of Succession when the monarch of one country or the other dies. Royal marriages are required for peaceful vassalization (which means that only monarchies can vassalize or be vassalized without the use of force).
Declaring war on a country with which you have a royal marriage will hurt your stability. Once a marriage is established it can only be broken at the cost of a stability point, until the death of a monarch.
- Main article: Claim Throne
Having a Royal marriage enables one nation to claim the throne of another. While this grants a Casus Belli, it destroys relations with both the target country and all other nations with whom the player has a Royal marriage.
As of HTTT, the claiming throne system has been overhauled. The target nation must now have a ruler with low legitimacy (the throne of a nation ruled by a regency council can also be claimed, but only if the underage king has low legitimacy). If the player has a Royal Marriage with the target, claiming the throne will give a Casus belli and allow the formation of a Personal union as a peace term.
It isn't possible to simply spam throne claims. Firstly, if you claim a throne, your relations with other countries you have royal marriages with will drop sharply. Also, eventually other nations will become wary of marrying into your dynasty. Claiming other thrones will also lower your ruler's legitimacy by about 20 points.
Make sure you don't try to tackle big targets. For example, as Poland in the early game it is very difficult to keep Lithuania in a personal union and the same applies with Denmark and Sweden. Try to get countries slightly smaller than yours but still significant in a personal union. This will mean you will benefit plenty, but won't bankrupt your nation doing so.