A mass combat system
This is by far the simplest Mass Combat system that I have found. I forgot where I found it. If someone remembers I would really like to credit the author. I think this works for most battles that the players will be involved in. It is pure 3e and uses all of the same rules everyone is familiar with. Basically, it is just a way to organize a mass combat. (Hopefully it is readable enough, some of the formating gets lost when you paste.)
Mass Combat Occasionally, battles will arise that are just too large to play out with the D&D combat system. For example, it can take a long time to resolve a battle when 40 goblins attack the PCs as they negotiate with a dwarven lord and his 15 bodyguards. You can use the mass combat rules whenever you feel they are necessary. Generally, if there are more creatures in the battle than will fit on your map, it's a good time to use the mass combat rules. It's usually best to leave PCs and important NPCs in place on the map and use the normal combat system to resolve their actions. Use the mass combat rules to determine the outcome of the larger battle raging around the PCs. Critical hits are not used in mass combat attack rolls.
Mass Combat Procedures
Keep paper and pencil handy to keep track of the troops on both sides. Start by dividing the opposing forces into groups of a convenient size; groups of 5, 10, or 20 usually work the best. If there is an odd number of troops, make one group shorthanded. For example, a group of 35 creatures could be divided into three groups of 10 and one group of five or seven groups of five. It is best to keep groups of similar creatures together. Assign each group an Armor Class based on the most prevalent AC in the group. If there is a tie for the most prevalent armor type, use the worst one. Assign each group a Base Attack Bonus based on the most prevalent Base Attack Bonus in the group, as above. Once you have assigned an Armor Class and Base Attack Bonus to each group, you are ready to begin. You may choose to check for surprise, allowing one group to attack unopposed, but after this combat is considered simultaneous. Assign one side of the combat to be the attackers, and one side to defend. Line up attacking groups against defending groups on a one-to-one basis. Each defending group must be attacked once before any defending group can be attacked twice. For example, a force of 60 orcs divided into six groups of 10 are attacking 40 dwarves divided into four groups of 10. Two groups of dwarves are attacked once, and two groups are attacked twice. The orcs can't attack one group six times. Note that in some cases only part of an attacking force can get into a battle. A force of 20 dwarves guarding a cave mouth might hold out against hundreds of orcs because only 20 of them can attack at a time. The attacking group makes a normal attack roll. If the roll succeeds, they will inflict one Hit Die on the defenders for each creature in the attacking group, ignoring critical hits; however, do not remove any casualties yet. Now, reverse the process, having the defenders return the attack, using the same procedure. When both sides have made all of their attack rolls, mark off enough casualties in wounded groups to account for all the Hit Dice inflicted, leaving only one wounded defender. For example, if a group of 10 orcs successfully attacks 10 3rd-level dwarves, three dwarves are killed and one loses a single Hit Die. Repeat this procedure until one side is killed or breaks morale. Apply subsequent hits to wounded creatures first. To save time toward the end of a combat, you can reorganize survivors into new groups. Creatures "killed" in a mass combat are allowed a fortitude saving throw (DC 10) if they receive some form of healing immediately after the battle. These figures are assume to be stable and can be saved. If there are a lot of casualties, roll the saving throws in groups of 5, 10, or 20.
An Example of Mass Combat
The dwarven leader and bodyguards from the example at the beginning of this section are talking to a group of six PCs when 40 goblins attack. The 15 bodyguards are 4th-level fighters (Base Attack Bonus +4), wearing chain mail +1 and carrying small shields (AC 17). The goblins have a Base Attack Bonus of +1 and an Armor Class of 16. They are armed with short swords and short bows. The DM decides to divide the combatants into groups of five, so there are eight groups of goblins and three groups of dwarves. The DM also decides that all the bodyguards will join the mass combat, leaving the PCs and two groups of goblins to fight a normal D&D combat system battle. The goblins gain surprise, and loose arrows. The ten goblins fighting the PCs attack individually. The remaining six groups attack the bodyguards, two groups of goblins against each group of dwarves. The goblins fire two volleys of arrows from 10 yards away. They are at short range and receive a +1 to their attack rolls because of surprise. Each group of dwarves receives four volleys of arrows (two groups of goblins each firing two volleys). Against the first group, the attack rolls are: 3, 17, 18, and 18; three hits. The first group of dwarves suffers 15 dice of damage; three dwarves are killed and one suffers three dice of damage. The attack rolls against the second group are 6, 17, 13, and 20; two hits. The second group suffers 10 dice of damage, killing two dwarves and inflicting two dice of damage on a third. The attack rolls against the third group are 3, 19, 4, and 14; one hit. One dwarf is killed and a second dwarf suffers one die of damage. There are no return attacks from the PCs and dwarves, because they were surprised. The next round, the PCs and the dwarven leader fight a normal battle against their 10 foes. They make short work of the goblins, killing seven of them. In the mass combat, the dwarves are now in the thick of the fight, charging at the goblins. The dwarves have a +2 bonus for the charge and an additional +1 because they are dwarves attacking goblins. The goblins are still firing their bows, and loose a volley of arrows as the dwarves charge. They receive a +2 bonus to their attack rolls because their opponents are charging. The dwarves complete their charge. Each group of dwarves attacks one group of goblins, while the remaining three groups are not attacked. Their attack rolls are 16, 17, and 19; three hits. The first group contains two dwarves who inflict two dice of damage and kill two goblins. The second group contains three dwarves, and they kill three goblins. The final group contains four dwarves, who kill four goblins. Each group of dwarves is attacked twice. The rolls against the first group are 3 and 18; one hit. The first group suffers 5 dice of damage; the wounded dwarf (who had already suffered three dice of damage) is killed and another dwarf is killed as well, wiping the group out. The rolls against the second group are 5 and 11; both misses. The rolls against the third group are 20 and 14; one hit. The wounded dwarf is killed and a second dwarf suffers two dice of damage. The goblins wisely decide to withhold their second volley of arrows, and the dwarves get no attacks of opportunity. Because there are only six dwarves left, the DM decides a morale check is in order, and the dwarves pass it. The DM also decides to re-combine the survivors into a single group of six. The goblin casualties result in 21 goblins remaining, which the DM divides into three groups of five and one group of six. The DM also decides that the group of six goblins leaves the mass combat to attack the PCs and dwarven leader, leaving three groups of goblins in the mass combat. (There are now nine goblins in the normal combat.) During the next round, the PCs continue to do well, slaying seven more goblins. The two survivors fail a morale check and flee the field. In the mass combat, the dwarves get two attacks this round, since they are specialists. The dwarves first attack roll is a 17, and six HD worth of goblins would be killed. This is enough to wipe out an entire group of goblins, but the sixth Hit Die will be wasted. The second attack is made against a second group of five goblins, and the roll is a 17; still good enough to hit the goblins' Armor Class of 16. Five more goblins will die, and the last Hit Die goes to waste. The three groups of goblins attack, rolling 20, 12, and 4; one hit. The attack kills two wounded dwarves (who each were two HD down) and wounds another for one die of damage. There are now five goblins facing the six PCs, the dwarven leader, and the four surviving guards. Finding themselves outnumbered, the goblins check morale and pass, not wanting to flee from their hated dwarven enemies.
The DM decides that the remainder of the battle can be handled using the normal rules, and the mass combat ends. The PCs and dwarves win handily.
