Personal Unions? How do you get them? How do they work? What do you do? lol
I see people on here talking about them all the time, how they "PU'd this dude, then PU'd this guy" and what not... How does it happen? What do you have to do? And what happens when you get one lol?
Best way of all: Get lucky and have someone you had a royal marriage with lose their ruler while they have no heir and you have high prestige, legitimacy, and relations with that country.
Awkward, inconvenient way: Have a royal marriage with a country that has no heir, or an heir with a weak claim. Use the Claim Throne diplomacy action, trashing your relations with every country you have a royal marriage with. Then either wait and pray they don't spawn a new heir, or declare war using the Claim on Throne CB and force the union.
Best reliable way: Open the first page of the ledger. Find a country in your religion group with low legitimacy. Use spies to Fabricate Claims, then declare war using the Obscure Documents CB and force the union.
Once you've got a personal union with a country, as long as your relations with them are positive you have a chance (depending on various factors including a heavy weighting on Trust) of inheriting them every time your ruler dies. While in a PU (whether as junior or senior partner) a country cannot change to a non-monarchic form of government. Additionally, any country you have a PU with will
always accept your offer of Alliance, and will always honor its alliance with you.
Also note that Noble Republics, while they can have royal marriages, can't have PUs.