This is going to be a briefer post than the others, largely because it does not involve number crunching. In fact, the purpose of this portion of the prep work is to reduce the amount of number crunching.
The battle and conflict as it’s statted out and scripted in the previous posts does not reflect the battle as a whole. While the 200+ Hit Dice of evil elven undead represents a formidable force more than capable of overwhelming the PCs and the fortress’s defenses, it doesn’t connote that “army” feel. It’s not big enough. Now, admittedly, this is going to be window-dressing, but it will certainly help the battle feel bigger.
Other factions –
Imperial expeditionary force – I’m not sure what all will have happened between the time I am writing this and when the encounter will happen, but one possibility is that Portsdam is destroyed by an earthquake. Whether that happens or things have just been so bad in the colony that word has gotten back to the empire, let’s say that an expeditionary force has landed north of Alfort and is on its way. Maybe 2000 strong, this force, while small will certainly distract a chunk of the Elf King’s undead army, preventing it from bearing its full brunt against the fort. If the PCs haven’t cleared out the Zombraire’s estate module, this force will probably be ambushed from west and arrive significantly weakened and unprepared to stand against Caelden’s army.
Eastern Goblin Coalition – The Southeast and Northeast goblin tribes have formed a military alliance. They understand that a limited human presence in Alfheim is preferable to the land being awash with undead elves. Sometime between now and when the battle is run, the PCs will be presented with a chance to dislodge the Northwest goblin tribe from the Old Island Fortress (if the PCs don’t go along with it, the goblins will later take this on their own). The Old Island Fortress will be used as a staging ground for the eastern goblin tribes to lend their support against Caelden’s army.
Northwest Goblin tribe – I’ve retconned my setting a bit to eliminate Orcs as an indigenous people of Alfheim; while Orcs are there, they’re mostly imperial mercenaries (note to self, the imperial expeditionary force should be comprised largely of Orcs). That said, I’m rewriting Starisel’s dungeon to be inhabited by goblins (with Orc stats) instead of the orc tribe. These will be part of the same tribe who were trying to take over Malek (the Nameless Dungeon) until they were slaughtered by undead. If the PCs can reconcile with these goblins (successfully run Cave of the Unknown), there is a good chance that they might be willing to commit to fight against Caelden.
So we’ve got a Battle of Five Armies, here, a perfect climactic fight for the campaign.
To incentivise the players to gather these allies, I might even take away the last two waves via some sort of plot-flash.