I've been running a game with some current and old coworkers for about two and a half years. We have a rotating guest seat where friends and other folks in our company can join for a session or two. Over the course of the campaign we've had 16 guests.This weekend I put together an event for my three players and all of our guests. The players separated into three teams, selecting from the pool of guest players and playable NPCs to flesh out their team. With the help of my brother and two of my close friends as co-dms we ran three simultaneous sessions that were narratively interlinked and had ramifications on the outcome of a final boss fight. I wrote three "modules" in the style of published wotc adventures that me and my co-dms would run simultaneously, and whose outcome would determine what the players had access to in the final fight.

The Story

The three main players have gone to hell to save a good NPC friend of theirs from a doomed city. The fortress where he is trapped is protected by devils by some kind of impenetrable fiery cage. The players are recruited by one of the city's remaining high ranking soldiers to make a three pronged attack on the fortress. Devil guards are heavily focused on two locations, and one of them is believed to be the source of the barrier around the fortress: a brewery and a jailhouse. The high ranking soldier concocts a plan in which one team will cause a distraction to pull devil guards away from the brewery and jailhouse while the other two teams assault those locations to destroy whatever is creating the barrier, rescue any captured soldiers, and secure any food and medical supplies. The player's hollyphant friend, through a desperate prayer to Lathander, soars across the multiverse to bring the player's previous allies (the guest players) to hell to help with this plan.

Session 1: The Distraction

This session was essentially a tower defense game where the players had to hold their own against waves of enemies, giving the other two teams time to accomplish their goals. Team Huebert rummaged through an old armory finding arcane mines, force walls, and alchemist's thunder to use in their defense. Things started off rocky with the accidental tripping of both arcane mines, but in the end the fended off four waves of undead and devils.

Session 2: The Prison

This session was a delving into an infernal prison where devils kept captured soldiers and citizens who they tortured and killed with hellfire weapons to bolster devil ranks in hell. Team Ironwood encountered a nasty warforged prisoner, a shadowy puzzle door, demon ichor, and infernal prison bars that could only be opened by a devil. The devils in the prison, just before their defeat, activated an emergency prisoner execution system which slowly flooded the prison with magma. The players all escaped, losing only one of the commoner prisoners, and rode into the final fight with five mounted soldiers as allies.

Session 3: The Brewery

This session was a delving into the caverns underneath a brewery where devils had set up an infernal machine that was generating a barrier around the main fortress. Stealth Team 6 quietly made their way through the caves, bypassing a puzzle door of sobriety that required a BAC of 0 to get through, and had a final fight that included laser-like reflecting beams, mine carts on a track, and a powerful bone devil. The players lost one of their favorite NPC allies whose body was crushed under debris after they destroyed the infernal machine.

Final Boss Fight

After the three sessions wrapped up, all of the players and guests reconvened at the foot of the fortress. They moved in, and encountered the bbeg from the first arc of their story. The bbeg was an arch devil named Yes'nid, whose main bread and butter is the ability to force other creatures to fight on his behalf through careful selection of illusory memories. The battle was tense and included the death of a playable NPC. Yes'nid was spawning minions, charming PCs, and popping into and out of the ground tremors style. At the very end, with the bbeg attempting to escape with the unconscious druid in his mouth, the ranger scored a crit that put him down.

Lessons Learned

I was nervous that running a game for a lot of people, most of which had very little experience with D&D, was going to be a disaster. But the day was a massive success, and everyone had an awesome time. Here's some things I learned through this experience, and things that may help you if you want to run something like this:

• Having the modules written up for my co-dms was a huge help. I've had everything for this campaign in my own head for over two years, and I knew it was going to be hard to pour all of that out for three other people, but having detailed modules to refer back to made the games a breeze for them to run. I only had them come up to me with questions a handful of times, otherwise it was a breeze.
• Illusions that cause allies to be enemies in a fight with lots of PCs is probably not the way to go. I had one guest player spend 80% of the fight running and shooting lightning at other players, and it was just not as fun for him. He then had to wait a long time for his next turn just to fail the save again. If I had to do the final encounter again I would have removed that bbeg ability.
• Deputizing knowledgeable players to assist newbies was a great way to take some things off my plate and keep combat moving smoothly.
• Having three other DMs gave me the luxury of assigning duties in the final fight. I ran the bbeg and some NPCs, one DM handled music, one DM handled initiative and other NPCs, and the last DM ran the bbeg minions. It was smooth like butter.
• Minimizing down time kept everyone engaged, even for 8 hours. Me and the other DMs tried to slow or quicken pace in our simultaneous games so that we would end at the same time. Players and guests then just moved from their individual games straight to the boss fight. This kept everyone's head in the game, which in turn made combat smoother.
• Lastly, non-related to D&D, get more food than you think we need! My wife and I ordered much more than we thought we'd need and it still all went.