Since an army's strength is represented by a CR score, you can balance armies against each other using the guidelines for CR in the Pathfinder RPG Core Rulebook. For example, two CR 9 armies should make for a relatively even battle, but so would a CR 9 army against three CR 6 armies. For War of the River Kings, the assumption is that the PCs' army will consist of a CR 11 army overall.
In most mass combats, the battlefield shouldn't impact either army. But sometimes a battlefield can decide the outcome of a war.
Mass combat generally takes place over the course of three battle phases—the Tactics Phase, the Ranged Phase, and the Melee Phase. A phase does not denote a specific passage of time, leaving you as the GM the latitude to determine how long any mass combat takes to resolve.
The following summarizes the key rolls in mass combat.
Offense Check = d20 + OM
Damage = Offense check result - defending army's DV + attacking army's damage mod
Morale Check = d20 + leader's Cha mod + morale score
During the Tactics phase, and each round during the Melee phase before rolling each Offense check, an army's commander must select a strategy from one of five options on the strategy track. Strategies adjust the army's DV, OM, and damage modifier (which is added to the damage dealt by the army but not to the offense checks themselves). Adjusting an army's strategy one step in either direction is automatic; if the leader wants to adjust strategy more than that in 1 round, he must make a DC 20 Morale check. If he succeeds, the army's strategy changes to the desired level, but if he fails, the army's current strategy doesn't change at all.
In mass combat, it doesn't matter who goes first, since in the game it's assumed that all attacks happen simultaneously. Each army makes an Offense check by rolling 1d20 and adding its total OM. This result is then compared to the target army's DV.
Attacker's Offense check equal to or less than defender's DV
Attacker's Offense check greater than defender's DV
If at any point an army's hit points are reduced to a number equal to or less than its CR, it immediately flees unless its commander can make a DC 15 Morale check. If this check fails, the army scatters and retreats from battle. When an army flees thusly, the enemy can make one final Offense check to get a parting shot before the army escapes.
These rules can also serve in battles where multiple armies clash. In such battles, when you make your Offense check, you choose which enemy armies you're attacking and apply damage appropriately—you can change targets each round as you wish—these rules don't take into account complexities such as movement or location, after all!
An army is victorious once all of its enemy armies are destroyed or flee the battlefield.
Leaders and risks
An inactive army heals back to its full hit points after a single month, but often, you'll need to restore your army to full fighting potential much more quickly. Each day that an army spends at rest (no movement and no battle), it heals hit points equal to its CR. Once per day you can also restore an additional amount of hit points to one of your kingdom's armies by making a successful Loyalty Check against your nation's Command DC. With a successful check, the army heals an additional amount of hit points equal to its CR.
To conquer an enemy hex, an army must remain in the borders of the hex for a full month. If the army has not been defeated within the month, the enemy kingdom loses the hex your army occupies and increases its Unrest by 1d4. All special resources remain in the hex and any Cities in the hex become Towns. The conquering army may then claim the hex like any other new hex during the Improvement Phase. If claiming a Town, the kingdom must roll a Stability check (DC = Command DC) and increases its unrest by 2d4 if that check fails (see Special Resources).
Alternatively, an army may raze enemy structures to the ground. To do so, an army must remain in the borders of the hex for one full day (in plains or grasslands), two days (in forests or hills), or three days (in mountains or swamps), without moving or entering battle. The enemy kingdom then loses the hex your army occupies and all enemy improvements in the hex (farms, roads, cities…) are removed. You may then claim the hex normally during the Improvement Phase. Razing an hex doesn't count as rest - an army razing an hex doesn't recover any hit points for the day.
As an option, you can have the PCs face battles of their own just before or even during a battle in which one of their armies clashes with the enemy. For example, the PCs might attack an evil necromancer and fight their way through his tower to confront and defeat him while their army battles the undead horde outside.
Alternatively, the PCs could use a few potent spells (such as control water, earthquake, cloudkill, and so on) to adjust the battlef ield condition to their favor. These possibilities allow the PCs to use their characters to directly affect the outcome of a battle without actually forcing the characters to "sit out" on an adventure opportunity by personally commanding an army.
If the PCs win their battle or dramatically affect the battlefield with magic, increase their army's DV and OM by +4. If the PCs lose their battle, penalize their army's DV and all OM rolls by -4. At your discretion, the PCs' failure or victory can have other effects on their army as well, such as granting temporary bonus tactics or starting the war with adjustments to one side's hit points.
Lyre of building
Inspire courage + Clarion call
Witch's healing hex