You can look at the loyalty of his vassals or who has claims to the throne and how strong they are.
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OP asked about DLC. I’d recommend these (I’d consider the highlighted ones almost mandatory.)
Holy Fury - vastly improves most pagan religions
The Old Gods - earlier start date and easier to play as a pagan as there are simply more pagan realms to choose form
Way of Life - easily the best bang for the buck given what it adds
Monks and Mystics - Adds secret societies, relics and heirlooms. Can get supernatural events with the demon worshipers. (Pagans can join warrior lodges and duel their way to the top. Very powerful stuff, especially if your liege has good military stats. You do not need Monks and Mystics for this though.)
Legacy of Rome - allows retinues (standing armies) which can be really strong
General advice:
Marriage - generally you’re marrying for stats, non-aggression pacts/alliances or claims. The smaller, less powerful you are the harder it is to secure a marriage. Daughters are good for alliances - the flip side is you can get dragged into wars and/or the other realm can now get claims on your lands.
Female heirs - you must marry them matrilineally. It’s easier to do this by inviting the potential suitor to your court. As ruler you get to decide who marries whom.
Succession - try to get out of gavelkind as quickly as possible. This form of succession can fracture your realm (there are ways to deal with it or you can just fight your siblings for their lands.)
Expansion (within your own Christian religion.)
You can expand your kingdom through conquest which requires a claim (there’s also de jure claims - basically lands that belong to your realm but owned by someone else.)
You can get claims by having your chancellor forge a claim. This is expensive and can be time consuming.
Inviting claimants to court and then pressing their claim. By far the ‘easiest’ method. Just be sure to grant the claimant a title first (and the title must be below yours - so if you’re a Duke, they can only be counts or barons.) Certain conditions have to be met before you can press the claim, but the tool tips will explain it.
Inheriting titles - without intervention, this is random. A lot of people “stab” through way the line of claimants (i.e. assassinating people in line to the throne you seek.)
The Pope can grant you the right to a claim, including entire kingdoms. Holy Wars also work and you can gain entire duchies and kingdoms through them. (n.b., everyone who belongs to the opposing religion can join against you.) Crusades can be very profitable with Holy Fury. Reforming pagan religions is really fun and you get to shape the nature of your religions Vikings are pretty good to play for new players since they’re relatively strong with the early start by the Old Gods. Tip - if you are pagan raid Christians. A lot. It’s your primary source of income.
All that said, once you get the hang of the game expansion isn’t all that difficult; generally I find it more fun to ‘roleplay’ and expand my dynasty. As always, YMMV
Damn how’d this post get so long. One last thing - always keep enough cash on hand to hire mercenaries should the shit hit the fan (invasion or rebelling vassals.) Managing vassals isn’t that hard but can be challenging for new players. Either keep them happy or keep them weak. Things tend to go smoother if they’re happy but have to be careful not to allow them to grow too strong. It’s important to develop provinces you control, not only for the additional income but for the troops you can raise.