Valairium is time limited. The Dark Lord will arrive in a certain year and that is the end of the game. In the meantime his servants, the Shadows, will keep arriving and spreading disorder to prevent any organised resistance to this arrival. Shadows are difficult to kill off, and the new ones that arrive with each new wave are basically more powerful then the ones before.
Conditions around the world will also deteriorate, the weather getting colder and more hostile.
The players will have a number of avenues to explore to solve the problem, but untimately they will need to defeat the Shadows and prevent the Dark Lord from passing over. To achieve this they can pursue several paths, possibly all at the same time.
They can gather personal power and confront the Shadows directly, killing them off. Several avenues exists for doing this, The Royal Lineage, the Artifacts, the various unique locations that will yield items of power. There are a finite number of Shadows, so tracking down and killing each one should be a significant event. You might make them very hard to kill, but easy to defeat.
They can gather an allied army of the land through the Pilgrimage and recapture Viriconium, destroying the Obsidian Tower.
Lastly they can summon the wizard Valarium, who will be able to defeat the Dark Lord, with the players help.
But they have to do this within the time frame you set, so you need to be very careful to leak out the urgency of the matter. At the same time you need to control the rate of advancement so that things dont happen too quickly. I have used 2 yearly intervals for the various waves, but one yearly works well also, just as long as your players know the time frames they have to work in, and you provide them with events at a timely pace.
The Orcs are essentially filler for all of the events, the muscle behind the brains. As far as the players are concerned orcs (whether Zord or God) are dangerous. You may find some of them helpful now and then, but essentially they are the primary physical protagonists. Orcs of God are more likely to be possibly allies, but in the end if you dont believe in their god you are expendable.
The best chances the players will have is to build up not only their personal power but to establish a set of relationships with the various groups they will encounter. Being able to call in a favour at the right time will make life a lot easier, but of course this means gaining favour initially.
Building a safe base of operations - South Hump - will also probably be an advantage. Early on there will be several attempts to wipe out the settlement and the players will have a central role in preventing these. Once South Hump is established it is in a good location for expansion moves.