My 13th Age Campaign based on the Dungeon World Front "The Great Wyrm Axtalrath" has taken on a life of its own. Which is perhaps to be expected, given the GMing philosophies and story game slant of the two systems. A few sessions ago the PCs traveled back in time to the 8th Age, encountering a very different Crescent Isle (and a very different Midland Sea, pre Archmage enchantment; wild and untamed). Their experience in the 8th Age planted seeds for overcoming the problems they've been facing in the 13th Age, the importance of some of which haven't been revealed yet.
Given the differences between the two ages, there's a lot of unexplained history and perhaps some avenues of interest that would inevitably lead to dead ends if pursued. So how did I address this potential issue? The almighty flashback. This past session the players took control of new PCs from the 8th Age (80 years after when the main PCs time-traveled to), all part of a crew tasked by the then-Emperor to take control of Crescent Isle by force (the native sea and wood elves took issue with the Empire essentially using their island as a prison colony, so they began sinking Imperial ships). The big upside to this flashback was also that the players got to experiment with other classes.
Fast forward to the final encounter of the evening. The party, backed by a platoon of imperial soldiers and 3 ballistas, took on the wood elf capital, their victory explaining why there are no wood elves on the island in the 13th Age. Oddly enough, it wasn't until I was a round or two into this combat that I thought "shit, I didn't even THINK about how the system doesn't have any mass combat rules!" Here's what I came up with, a few rounds into the combat (before that I described everything narratively).
I kept track of different units or sections of the battlefield. The Ranger was fighting with the soldiers, but the Wizard and Rogue went off on their own to take out some Druids that were controlling magical blasting stones that were decimating the soldiers and damaging the ballistas. Archers on top of the city walls were raining death down from above, while Druids wild-shaped into bears and wolves were trying to kill all the soldiers in the vicinity of the ballistas to neutralize them. Soldiers were escorting companions carrying a battering ram, and later in the fight I had a second unit arrive with reinforcements.
Perhaps appropriately since the idea for the campaign came from Dungeon World, my dirt-simple, on-the-fly mass combat rules were also inspired by Dungeon World. The core mechanic in Dungeon World is roll 2D6+modifiers, with a 6 or less as a failure, 7-9 as success but..., and 10+ as full success. Ok, so technically you could argue that the core mechanic of Dungeon World is actually narrative description, but when the dice come out that's how they roll.
Using that as a base, I rolled a d6 for each unit and on a 1-2 bad things happened, on a 3-4 both sides had some success and it mostly evened out, and on a 5-6 good things happened. It worked well enough to give the impression that I wasn't just making up whatever I felt like, but it was still pretty simplistic and didn't account for relative unit strength. Clearly there's room for improvement.
Refining the Basic Concept
Still, I like the base of letting large-scale events hinge on the roll of a d6, with results being more than just binary success/failure (namely that those middle values are a partial success). The question, then, is how to assign value to units in the simplest way possible?
I'm thinking of using the strongest unit present at the outset as a baseline, and giving it a value of 10. All other units are assigned values as a percentage of the strongest (a unit half as strong would be given a value of 5). Unit strength is an abstraction, so a unit with fewer raw numbers but in a highly defensible position would be pretty strong. An eyeballed approximation of advantage. This is still meant to be quick and dirty, potentially even run on-the-fly.
A unit's value represents its "damage roll." A multiple of the unit's value (10?) would serve as its "HP." Rolling for just a single side (the player's side), a 1-2 would deal the enemy unit's damage to the player unit's HP and the player unit would deal half damage to the enemy, 3-4 both would deal their damage to each other, and 5-6 the player unit would deal damage to the enemy while the enemy only deals half damage. Just like a group of mooks, once a unit takes damage equal to the HP multiplier, its value is reduced by 1.
Once a unit's value is reduced to half of the opposing unit's strength, that unit would start making morale rolls. I'm thinking a d6 with 1-2 as a failure, with failure increasing by an additional 1 for each additional point below half. If PCs want to act as "commander" types they could make Cha checks (with any relevant leadership background) to provide a +1 bonus to morale checks. Or maybe even damage checks.
For the sake of simplicity, unit strength could be tracked using d10s. If you use a map, the d10 could even represent the center of the unit's position.
Anyways, this hasn't been playtested AT ALL, it's mostly just me throwing out an idea for simplifying mass combat as much as possible. It's also worth mentioning that the way I envision this, it probably isn't the main focus of the combat. The PCs are off fighting specific foes at critical locations using the normal combat rules, with the mass combat going on in the background. So anyways, give it a try, tweak it, and let me know how it goes.