I love the concept, but the logistics of running that battle just make me want to put my face in my hands and sob.
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Without actual mass combat rules that differ from normal skirmish-scale combats, that's not a fight I want to run!
-O
This may not work for either of you guys but I found this worked extremely well for an epic end of heroic tier fight against a barbarian horde that was attacking a small winter settlement that the PCs were defending. I did a few things here:
1) I narrated the settlers small guard dying valiantly beside the PCs within the horde's swarm of seething flesh and steel while protecting the city. The settlers guard were a small swarm themselves with an encounter power. They died by the end of the 3rd round (as predicted) but they made a small dent in the horde's HPs and provided aid by way of the encounter power (Immediate Reaction upon their noble sacrifice giving the PCs temp HPs - basically inspiration/morale).
2) The terrain/battlefield was pretty interesting as well with some Limited Use Terrain (Ruined Wall, Snow Dune) and the field-stone wall and guard tower was climbable to escape the aura (but triggered an OA due to the swarm trait). Better than that, I ran a Skill Challenge beforehand where the PCs and the settlers attempted (successfully) to set up some Limited Use Hazards (in the very limited time - one evening) before the fight to prep the battlefield. They made full use of their efforts once the fighting started.
3) After the Barbarian Horde Swarm was defeated, I split it up into the lvl 10 Solo Warlord (Primal Controller, Leader) and 6 lvl 11 Minion Guards (2 Soldiers, 4 Brutes).
It captured the feel of an implacable horde sweeping down upon a small settlement and the desperate fight to fend them off quite well. This was only 3 PCs (one controller, 1 striker, 1 defender/striker/controller hybrid), their strategic setup of the town before the raid and the small group of the settlers' guard as defenders. I counted the settler swarm + limited use hazards as an extra PC so for a 4 party group this fight was almost exactly L + 5. I tested it a few times beforehand to make sure things worked as I wanted and it worked out better than any "Mass Combat" I had ever run before. It captured the dynamism, the desperation and the climactic nature I was hoping for (it was basically a 50/50 TPK). Below is the Swarm I created for it (feel free to rip it for your own campaigns if it seems something to your liking):
Barbarian Horde
Colossal natural humanoid (swarm)
Level 11 Elite Brute XP 1200
HP 276; Bloodied 138 Initiative +9
AC 23, Fortitude 24, Reflex 23, Will 21 Perception+7
Speed 5
Resist half damage from melee and ranged attacks; Vulnerable 10 to close and area attacks
Saving Throws +2; Action Points 1
Traits
Swarm Attack * Aura 1
Any enemy that starts its turn in the aura takes 7 damage and a -2 penalty to all defenses.
Swarm
The horde can occupy the same space as another creature, and an enemy can enter its space, which is difficult terrain. The horde cannot be pulled, pushed, or slid by melee or ranged attacks. It can squeeze through any opening that is large enough for at least one of the creatures it comprises.
Standard Actions
Reckless Abandon * At-Will (Martial, Weapon)
Attack: Melee 1 (one creature and each creature in the mob’s space); +16 vs. AC
Hit: 2d8 + 13 damage, and the target is knocked prone.
Move Actions
Flowing Horde * Recharge when first bloodied
Effect: The mob shifts up to twice its speed.
Triggered Actions
Trampled Underfoot * At-Will
Trigger: An enemy in the swarm's space is knocked prone.
Attack: (Free Action): Triggering enemy; + 14 vs Fortitude
Hit: The target is restrained and takes 10 damage (save ends)
Warlord's Curse * Recharge when first bloodied (Implement, Primal, Psychic)
Trigger: A close or an area attack hits the mob.
Attack: (Immediate Reaction): Ranged 20 (triggering enemy); + 14 vs Will
Hit: The target is weakened and takes 10 psychic damage (save ends)