First, you have to arrange that the players have contact with all the various elements that will be involved in the great conflict over the course of the campaign. That way they know what to expect when the see the various components hit the field. It also can give them a personal tie to the conflict. See the difference between “The White Elves Cavalry take the field. Their formations comes wheeling about to hit the Orcish line,” vs “Perrignor, and the handful of White Elves lancers, who were the rear guard of the Elves travelling to The West, come thundering on to the field, wheeling about in formation to hit the Orcish line”. The players realizing that these warrior have given up their chance to join their brethren in The West to fight this battle. This gives them an understanding of the various forces involved and can add much to their experience.
Secondly: Brush up on your military tactics and history. If you are going to have a grand battle, you need to know how such battles are conducted. In addition to giving you an idea on the hows and whys, this brushing up will increase your appropriate vocabulary, so you can describe the action in military terms. I read up on Samurai Warfare for months before the The Three Grand Battles, involving 26,000 Samurai, we resolved. I watched Ran the day before. It was glorious.
Third: The GM has two options when dealing with war: scripted and non scripted. A scripted war is an event planned by the GM, acting as the author of the game. The advantage of this is that the GM can plan adventures around the events and control all the changes. The disadvantage is that the players have little to no impact on the events. An unscripted war is one where game mechanics are used to determine which side wins which battles. This is vastly more fair, but oftentimes does not provide a good “story” for the adventurers. Note to GMs: Have your players play out some battles, using the rules of your choice. Use those results to determine the results of an upcoming war in the game world.
Now I am a big fan of the nonscripted battle. However, I will admit to giving certain plus modifiers to one side I felt should win the battle despite their game mechanic deficits. While I refused to script the battle, I felt justified in fudging the rules a bit for the betterment of the campaign.
Forth: Know your battle mechanics cold. You have to be totally proficient in the mass combat rules you are using. We have all played with GM’s who have not known the character level combat system. We have all experienced the halting, jerky flow of said combats. You do not want that for your grand climax. So practice the mechanics, even play out a few mock combats with other players ahead of time to get fluent.
Forth and a half: Know the terrain of the area your grand battle will take place. Have a map set up for the area.
Fifth: Make sure that the characters are involved in the combat in some way. Often times players are not where they expect when these combats occur, so be prepared to keep them involved.
Sixth: If your mass combat mechanics do not have a character scale integration (Like Bushido), find a way to do it. You need to get them involved in the mass combat, on a character scale.
The easiest general mechanic is make three “attack” and three “defense” rolls. This determines what generally happens to you (attacks determine number of kills, while every missed defense roll is a die worth of damage). Cheesey, but workable.
The way I suggest (having done it in a few other game systems) is having a handful of “in battle” scenes all set up on cards. Every turn I flip a card and the players roll a die. On the card, it explains a battle field situation that will come up… A personal combat, the character is some how surrounded, the rain begins, encounter a leader, etc. These “scenarios” are often modified by how the battle is going, (surrounded by enemies when you are winning is you just being temporarily cut off, while if your side is loosing means you are gravely outnumbered). Some cards effect everyone (the rain begins and continues for 6 rounds… make dex checks to keep weapons or footing every turn). The High or Low roller is the one that is involved in the scenario. Note: The players will often say that they are sticking together, so they can sometimes draw more players into said scenario. We resolve the scene, and roll the next “Battle Round”.
Seventh: Be prepared to play the post climax scenes. Don\‘t get too sticky about timing or details. Let the scene be dramatic and personal.
Eighth: Be prepared to do this all in one session, so plan an extra long, marathon session with food and drink to sustain the group. If you break up these climactic moments into two sessions, much of it is lost.
Ninth: Get everyone into the same mood and groove as you. While you are brushing up on your mass combat skills, see if there is a movie or three that has a large combat in the same time period or genre. Have a film festival before the final event, so everyone is “visually” on the same page.
Tenth: Make sure the players know what they need to do in a mass combat (and how to use those rules). Even if they don\‘t actually use them before hand, make sure they are familiar.
That should be it for right now. You know I will post more when I think about it some more.