Mass Combat

 This is a system designed for large scale combat, specifically between smaller sized armies. This won't emulate thousands on thousands, but would emulate a few hundred to a thousand odd soldiers on each side (think the Black Company). This system likely becomes extremely unwieldy after 10+ units on each side.

Units

Units come in 5 types: Infantry, Ranged, Cavalry, Siege, and Heroes. These are loose terms, treants are infantry just as axmen are.

Each unit is defined by its name, its health, its damage, and any special abilities. For example:

• The Fourth Vanguard of Poles (Infantry): 8 hp, 1d6, +2 vs Cavalry
• Goblin Rust-Arrows (Ranged): 4 hp, 1d4
• The Grain Knights (Cavalry): 12 hp, 1d8, Unbreakable
• Seedmark's Onagers (Siege): 4 hp, 1d6
• Malicrag, the White Dragon (Melee Hero): 12 hp, 1d8, Can attack anything, Flees at 4 hp.

How units are acquired is up to the GM. 

Combat

Before battle starts, the commander of each force decides their intended actions for each form of unit according to the following rules:

• Siege units attack anything before battle, but cannot fight during battle.
• Infantry must attack other infantry, unless there are no opposing infantry in which case they can attack anything. Infantry can also defend a target, taking attacks instead of it (regardless of which side goes first).
• Ranged may attack infantry, ranged, or applicable heroes.
• Cavalry may attack anything.
• Heroes may attack anything applicable to their type.

Afterwards, combat is started in the phases of Siege (First Round Only) > Ranged > Infantry >  Cavalry.

During each phase, each commander rolls plus their charisma (or attempts a Charisma check, or whatever fits for the game) to determine which force acts first. Units that are attacked are stuck in combat, and cannot perform their intended actions, instead attacking one of the units they are engaged with. IE: if you send your infantry to attack a certain unit, but they are attacked first by a different unit of infantry, they are stuck fighting the second infantry. Attacks always hit.

After initial engagements, units alternate attacking a unit they are engaged with, and ranged units firing at valid targets, unless a unit is engaged with them, in which case they fight in melee (ranged units fight in melee with a d4). If infantry or cavalry becomes free of engagements, they may engage a new enemy, abiding by their regular rules.

When the tide of battle shifts dramatically against a side, all of its units must make a morale check (2d6 attempting to beat their health). Units that fail are removed from combat. Unbreakable units never flee.

Post Combat

Afterwards, damaged units may regain 1d4 hp, up to their max at the start of combat. All other damage taken is permanent. Units with 0 hp are fully lost, though units that fled due to failed morale are allowed to return to the army and heal.

Example!

The Shields of Wrenn (Led by Galian, Savior of Wrenn) are a mercenary company charged with protecting a town from an aspiring necromancer and his horde of undead minions. The Shields have access to the following units:

• Wrennish Militia (Infantry): 4 hp, 1d4
• Long Mountain Dwarves (Infantry): 8 hp, 1d6, Unbreakable 
• The Rangers of Elding Lodge (Ranged): 6 hp, 1d8, Fights in melee with a d6.
• Galian, Savior of Wrenn (Infantry Hero): 19 hp, d8, 3rd level fighter

The Necromancer has access to the following units:

• Zombies (Infantry): 8 hp, 1d4, Unbreakable, Is defeated when their necromancer dies
• Skeletal Knights (Cavalry): 8 hp, 1d6, Unbreakable, Is defeated when their necromancer dies
• The Necromancer (Siege Hero): 4, 1d8

The Shields decide that their best course of action is to take out the necromancer, which means defeating his undead first, thus splanning to send both units of infantry to attack it, as well as their archer, and Galian.

On the opening round, the necromancer goes first (as he's a siege), and strikes the Rangers of Elding Lodge for 2 damage, dropping them to 4 hp.

Moving to ranged attacks, the rangers go, pelting the zombies for 2 damage and dropping them to 6 hp.

To infantry, the necromancer beats Galian in a charisma test, and the zombies charge into the Wrennish Militia, attempting to destroy them before they can attack. They hit for 3, and the GM rules this is scary enough to cause the militia to flee. The dwarves and Galian then engage the undead, easily defeating them.

Finally the skeletal cavalry attack the rangers, engaging them with 5 points of damage, easily defeating them. Were Galian not played by a foolahrdy player, and the Dwarves of the Lone Mountain not unbreakable, this would likely the end of their battle.

Luckily, it isn't, and on the second round of combat the dwarves and Galian assault the necromancer and easily deal over 4 damage, ending the battle.

Afterwards, the milita are patched up for their missing 3 hp, their wounds more psychological than physical. Sadly, the Rangers of the Elding Lodge have met their last hunt, and are given a proper burial as a respect for their service.