The Wayfarer's Inn

Here is a quick and efficient combat system that I devised. It is quite simple and most of the time works correctly. Of course there are exceptions and the DM should use common sense when using it.

It is NOT a mass combat system devised in order to turn a D&D game (or Ravenloft game) to a strategy game. It is more or less a quite accurate tool for the DM to quickly see how for example the Unit lead by the PC party fares in a battle.
Or even better, so the DM could let the dice quickly decide how the battle progresses while he concentrates his attention on the players actions.

So here it is:

1> First you calculate the Equivalent Level of each unit involved. I.e A group of 30 1st level human warriors and 4 2nd level officers is EL 9.
A group of 200 1st level human warriors, 10 3rd level officers and 4 6th level captains is EL 14

2> Calculate Battle Rating for each group. The BR starts as ELx2 + modifiers. Each condition the DM thinks is favorable/unfavorable gives a +1/-1 to the BR of the group.
I.e if the group has EL 14 and is:
Attacking the enemy from an ambush (+1) and their leader is a great strategist (+1), fighting within sight of their homes (+1) in bad weather (-1), tired from a long march (-1) the total BR is 14x2 +3 -2 =29

3> Roll initiative: Each group rolls for initiative, using the average (or better median) initiative bonus of the group. For example if Group A is elven soldier all with Dex 12 - 16 then the average initiative bonus is +2

4> The battle: The group that goes first decides a target and attacks. Roll 4d6+ the group's BR vs a DC of 10+the enemy's BR. If the roll succeeds the target's BR is reduced by 1. The target usually retaliates immediately after the attack using his new BR.
Proceed with the second group in the initiative order etc.

5> End of the battle: A group that cannot efficiently keep on the combat (i.e needs 22 or more in the 4d6 roll to cause damage) is effectively beaten. Remember that if the BR drops by 4, half the unit's soldiers are dead, captured, dying or otherwise neutralized. Remember also that Soldiers usually flee losing battles instead of fighting to the death. They may join other groups or run for the hills.
As the DM you should decide what happens.
For example a group of humans defending their homes may try to abandon their positions and flee to join another group of defenders, rising its BR. If they have nowhere to die, they may fight to the death. On the other hand, if the attackers lose half their numbers (i.e their BR drops by 4) they may decide to retreat.

Special rules:
- If a unit disbands/retreats while engaged in melee with another group is subject to an attack (like an attack of opportunity) but doesn't retaliate.
- A unit that can't harm its opponents just receives damage without retaliaton or attack! I.e a group of 100 humans without silver weapons against 15 werewolves do NO damage. The DM of course could rule that if the creatures in question have low DR that the unit has some chance to overcome it, applying modifiers accordingly.
- A unit that holds the terrain after the battle, restores about half its fallen. It may decide to show mercy and restore half the enemy's forces too, or save them and capture them. They may decide to kill the wounded.
- Each round of the battle doesn't have a fixed amount of time. Each round of battle between a group of 35 humans and 9 ogres may take 4-5 rounds, while a round of battle with 15000 combatants in each side may take 1-2 hours!
- Remember: BR -1 is about 15% casualties, -2 is 1/3 of the unit in casualties, -4 means 50% of the unit are dead/captured/wounded, -6 means 2/3, -8 means 75%. The winner usually manages to heal 25-50% of its casualties.
- A group that can't reach its enemy is considered unable to attack/retaliate. I.e 100 soldiers without bows are powerless against 30 harpies with bows.
- The DM may rule that the group gets a "free" attack against an enemy if the unit has ranged weapons and the enemy doesn't as they pellet arrows/javelins towards the enemy as he approaches.
- If the PCs are involved and do their part, calculate their EL with the group. If they do exceptionally well, increase their EL if they don't use their powers/don't get involved much, reduce their EL.
I.e a 7th level wizard using a couple of fireballs and a scroll of fireball, is doing his part.
A 7th level wizard that uses a magic missile spell and miscalculates the fireball getting only a couple of soldiers missing the leader, and then is downed by crossbowmen, is not doing his part.
A 7th level wizard emptying a fresh wand of fireballs on the enemy dealing death like the fist of God, protected by "protection from arrows" and three rows of pikemen so none can harm him, is doing more than his part.

Clarifications:
- If the PCs do something worthwhile in the battle like killing the enemy general or his three death knights, or even manage to rout/destroy an entire group of soldiers don't be affraid to add modifiers in the BR or even reduce it.
I.e if the PCs manage to kill the three Death knights leading an enemy group with EL 13, and the CR of each DKnight is 8, then the EL of the group immediately drops by 2. The DM may further decided that the BR drops 5 instead of 4 since the enemy is demoralized!

- I propose 4d6 +BR instead of the usual d20+BR because the 4d6 have a range of 4-24 and most importantly a bell curve, meaning that it is more probable to get a results around 14 than say 5, 20 or 24.