In many ways, the Kingdom of Adunabar is a mirror to the Reunited Kingdom. They occupy lands opposite each other at similar latitudes, with the RK standing for the tradition of Elendil and his heirs, while Adunabar has recently embraced the dark influence of the Shadow Cult. It's tempting to see Adunabar as “the Enemy” in the Fourth Age.
And to be honest, there are good reasons for that perception. In addition to the general creepiness of following a Shadow Cult, Adunabar's leaders can employ an entire roster of Cult-influenced soldiers, from low-tier spearmen to elite Knights of the Dark Tree, and everything in between. These units wear their black-hearted allegiances on their sleeves – and if having Mannish Cultic troops isn't enough, the player of Adunabar can embrace his dark side by recruiting Orcs, Uruks, Wargs, and Trolls.
But, in keeping with FATW's general policy of giving the player options, Adunabar doesn't have to be played that way. Although Adunabar begins play with Cultic temples in most of its settlements, it is possible for a player to turn to the Ways of the West – forsaking the aid of Cultic troops and bestial allies. In their place, Adunabar would be able to recruit a full roster of non-Cultic Dunedanic soldiers unique to its kingdom. These include some specialized, Adunabar-only troops in addition to stalwarts such as the Royal Spearmen and Longbowmen we remember from TNS.
But why would anyone choose to give up the power of Olog-Hai or the mass of bands of Uruks in order to “go good”? For one, the Shadow Cult is unwelcome in... well, every part of Middle-earth except your own homelands. Turning to the Ways of the West would ease the conquest of Gondor and Arnor, as well as lands owned by factions such as Rohan, Tharbad, and Rhovanion, all of which you are likely to come into conflict with. Ways of the West won't earn you any popularity in Rhun or Harad – but then, neither would the Cult.
On the other hand, a self-inflicted conversion means you can't use that difference in outlook to cause trouble in your enemies' lands. There's a sneaky kind of satisfaction in sending agents to convert the populace well behind enemy lines, causing heaps of unrest and – ideally – a rebellion that forces your foe to spend resources quelling a Cultic uprising.
But playing as a non-Cultic Adunabar also allows the player to explore the mindset of a proud, Dunedanic ruling family that sees itself as the “true” heirs of the great Numenorean kings of the Second Age. Ar-Pharazon the Golden brought Sauron to submission through the might of his Dunedain alone. Playing as Adunabar allows you to exercise a similar strength of arms and claim all the lands of Middle-earth.
Of course, having a claim is one thing – enforcing it is another. Adunabar owns a great deal of territory at game start, but that territory is located in the center of the map, surrounded by factions who are at war with you – or soon to be.
Like the Reunited Kingdom, Adunabar's strongest presence is in the south. The mountain-ringed fastness of Mordor is mostly easy to defend, but its east is exposed to invasion from the men of Rhun and Khand. Protecting the fertile lands around the Sea of Nurn is crucial, and requires constant vigilance – and preferably an army stationed in the area to hold off any Easterling aggression. Diplomacy may stave off threats from this direction for a while, but once the various eastern factions have finished squabbling for supremacy among themselves, expect a strong victor to come calling. To avoid this scenario, you may need to involve yourself in the politics of these barbaric chiefdoms sooner than you'd like, by attacking emerging threats before they become too strong. Extend eastward too far too soon, however, and you leave your heartlands vulnerable...
Meanwhile, your western forces will be clashing with the Reunited Kingdom around Osgiliath. Expect no peace from Rohan while the war with your rivals continues – and expect that war to drag on to its bitter conclusion. (That conclusion being, preferably, the reduction of the Reunited Kingdom to an ironically-named footnote of history in the annals of Middle-earth – which will earn you some rich, fertile lands and valuable trading ports along the Sea, as well as another defensible region protected by mountains.)
To the north of Mordor, there are unclaimed lands in Rhovanion, but the faction of the same name is interested in claiming them for itself. Wait too long to expand in this direction, and they – or another faction, such as the Beornings or Rohan – will claim them. While that may not seem like a problem, in the long term it means you'll be defending your mountain ranges for decades while your other armies advance in different directions.
And to the south of Ithilien, the tiny Principality of Harondor is all that stands between you and the Empire of Harad. Harad, of course, will be entangled in wars of its own, and its hatred of the Reunited Kingdom will mean that you two may make good allies in the early years. But once the Empire has dealt with the thorn in its side that is Harondor, it may view your kingdom as the next big threat in the area. Do you capture ports and send fleets down the coast, or hold off the waves of Southrons at the Poros until you can spare the men for a proper assault?
If a potentially 4-front war (!!!) isn't enough, Adunabar's holdings in the North open up other possibilities for conflict. When the civil war broke out within the Reunited Kingdom, many lords in the eastern and southern parts of Arnor declared for Adunabar. This puts you in charge of lands stretching from the Misty Mountains down the Greyflood toward Tharbad, and ensures warfare in the North between your forces and those of the RK. But other peoples also live nearby – the Elves and Dwarves, the Shire-folk, and the men of Tharbad, as well as numerous Orc-dens in the lonely foothills and mountain sides. Across the Misty Mountains themselves, the Beornings and the men of Dale have settlements nearby, separated from your lands only by a few narrow passes. Though wars in the North will be on a smaller scale than wars in the South thanks to a lower population, they will be no less lively, and full of just as many opportunities for backstabbing and betrayal.
A quick review of the above will reveal that Adunabar has very good odds of bordering almost every faction in the game within a very short period of time. That gives you lots of opportunities for expansion, but also plenty of danger that you - as one of the largest factions, with the added liability of religious and diplomatic baggage - will end up a target for many of your opportunistic neighbors. (Good thing you've got one of the finest military traditions in Middle-earth to draw upon.)
So, while Adunabar may see itself as a rightful inheritor and ruler of Middle-earth, the reality is that any claim to such inheritance will be hard-fought, and require a combination of ruthless expansion and patient defense. There are simply too many possibilities even in the very early game for me to sketch out a one-size-fits-all strategy here; you'll need to decide where to defend, and where to attack – and when – in order to see your Cultic legions (or Dunedanic royalists) raise the banner of the star in every corner of the world of Men.