How to end a level 20 D&D campaign with a bang

-----UNDERMOUNTAIN SPOILERS-----

-----HALASTER BLACKCLOAK SPOILERS-----

I've been playing and DMing D&D since the late 70s. Most of my campaigns have been homebrew worlds with my own rulesets. But as with many of us, the pandemic became a personal golden era of online gaming with friends around the world, especially with the old classic modules and Roll20.

This campaign began simply enough. It was called Thug & Thugger, and it only had two players. I told them they were thieves who stole from other thieves, interrupting the thefts and taking what they wanted for themselves. And it worked fine for the first 7-8 levels. But then we got ambitious and I sent them on a Spelljammer ship into the Phlogiston. There, they found the protection of an elder goddess who had been imprisoned and needed them to rescue her. But in the course of their rescue, things went sideways and instead of being murdered by Nalfeshnee and Hezrou demons, the elder goddess in a last gasp to save her heroes sent them "somewhere random."

Where they landed was the seventh level of The Dungeon of the Mad Mage in Undermountain. Not only that, but the demons had been compelled to be their familiars and... once they figured out their scale issues... all of them were no more than nine inches tall. The characters discovered that they couldn't go up any floors, only down. If they were going to survive this, they would have to conquer all 23 levels of the dungeon. Now, I'm well aware that this plot sounds like it came out of the diary of a 12 year old, but ultimately what we wanted was a campaign that finally took players all the way to the end and allowed them godhood after level 20.

With all our play these last few years, the schedules of daily life had defeated nearly every campaign. That was why we only had two players. And that was why we shoe-horned our narrative arc into the only module we could find that would get the players to 20.

The first five floors or so were an absolute bloody blast. They were immensely overpowered, despite being only nine inches tall, and they went through entire hordes like a buzzsaw. After the near-death challenges of the Phlogiston it felt like a victory lap. And as the DM I was fine with it, knowing their bully ways wouldn't last. At a certain point, one of their foes banished the Nalfeshnee (which was a massive loss--those things are stupidly powerful) and they regained their former physical height.

Then it was a fight. The two characters were a warlock/bard and a ranger/monk. Both fought well in the magical dark without disadvantage. That was their main strategy: cast darkness and then wade in. It worked for most of the levels and against a wide variety of enemies, especially since the vast majority of spells require you to "see" your target. But then the monk started spamming stunning strike and they got back to running the table on me. Dungeon of the Mad Mage was written before stunning was a thing, so not a single foe had resistance or immunity to it. He would burn through the legendary saves of nearly any bad guy and still have extra ki points left over.

They leveled and leveled again. They also became clerics to appropriately worship the elder goddess. Their bag of holding filled with gear and each action or attack became as convoluted as a Disney contract. The number of saves, reactions, buffs, etc. that needed to be accounted for on every move was something I won't attempt again without an AI assistant. As they approached the final battle, I realized that I needed help.

One player had told me that another friend group of ours had tried the year before to take on Undermountain but that campaign had fallen apart. So I secretly texted the DM of that group with a proposal: Since you know this campaign so well, I need some assistance for the end. He happily agreed.

On the day of the final battle, the players were locked in combat with Halaster's most senior minions. Suddenly, the Mad Mage himself arrived. That's right. On the Zoom channel, someone new joined. Someone named Halaster. He appeared in a wizard robe and fake white beard, wielding a scepter he'd bought on Amazon, with a screen behind him generated by AI to look like Halaster's lair.

My players lost their minds. They thought I was just going to put on a corny voice and be Halaster myself. No no no. But this wasn't to be just a cameo. I told the new player to legitimately have Halaster kill them. I wasn't looking for a happy ending. And as the DM I wasn't going to be anything but the referee, adjudicating what had now become a PvP situation. Two players against the Mage. My two players finally realized what I had in store for them. This was a serious no-holds-barred fight to the death.

The Halaster player is also a legendary game designer in his own right, a video game designer turned executive who has worked on many games we all know. He called in several other legends of the industry to help him figure out his moves. I even handed him the gift of cursed gloves I'd tricked the monk into putting on several levels before, which made his stunning strikes against Halaster something he needed to roll on the wild sorcery magic table.

And they still beat him and won their freedom and the freedom of their elder goddess. But man was it a battle. They withstood his meteor storm and made saves against his most potent spells. At the end, the bard only had 3hp and nearly everyone else was dead.

But we did it! We finally finished a level 20 campaign. And now we know we never need to do that again, lol. It became so unwieldy and slow after about level 15 that it felt more like work than play.

We look forward to starting over with simple characters who do simple things. The monk will be the DM this time, leading me and the other player in the Lost Mines of Phandelver. And each of us will try playing two simultaneous classic characters this time: me a dwarven cleric and elven illusionist, he a half-orc fighter and wood elf rogue. At least we know our schedules work.