Night Below: An Underdark Campaign is a classic AD&D adventure that I purchased when it came out in the 1990s but which I never got to run all the way through until the 2010s. Beginning with D&D 3rd edition and eventually converting to Pathfinder, my final version of the campaign saw some changes, including revising the Rockseer elves and adding a secret villain behind the aboleth conspiracy: the Red Mage.
Return of the Red Mage
After several years of gaming, the campaign ended in a way that I found quite satisfying. The PCs defeated the aboleth in a series of epic battles that taxed them but didn’t destroy them. When the Grand Savant finally died in an explosion of slime and gore, there was a moment of peace. (Well, technically, that moment had to wait until after three of the party got cured of the aboleth slime that took away their ability to breathe air.)

As the dust settled, I gave the players an option: end the campaign now or keep going forward with an adventure that would take them to around 20th level and quite possibly beyond. They chose the latter, so the Red Mage stepped back in and asserted himself as the real bad guy.
Between this campaign and the last time the Red Mage appeared, 100 years of game time had passed. During that time, the mage had been in a self-imposed temporal stasis effect, hibernating and waiting for all his old enemies to die out. He awoke when some mind flayers ended the effect and sought an alliance with him. He wasn’t interested in any alliance and certainly had no desire to see somebody else take over the world, but he also wasn’t going to throw his life away attacking the aboleth head-on. Luckily, after meeting the PCs, he decided that they were the folks for the job.
A Bad Deal
For most of the game, the Red Mage had been lurking in the shadows, watching the heroes and making sure they were on course for victory. His presence showed up only in ways that the players wouldn’t have picked up on, such as the fact that a group of mind flayers they allied with were able to teleport.

In entering an alliance with mind flayers, the PCs were wise to protect themselves via a contract. This contract ensured that they would not be betrayed, that the slaves of the aboleth would be freed, and that the mind flayers would not ally with “the red one.”
That last bit was the sticking point.
Mind flayers don’t really see much difference between humans. They see people about the same way that we see cattle. So by defining the Red Mage only as “the red one,” the mind flayers knew they weren’t supposed to ally with a guy based on the color of his robes. So when the Red Mage showed up dressed in black and offering an alliance of his own, the illithid decided it was technically within their contract to do so. And so, after the death of the big bad of the campaign, the mind flayers showed up again with the Red Mage (now dressed in black) in tow.
Whether this was a legitimate out for the contract or just me dicking around is a matter of debate. Had one of the PCs gotten a bad sunburn, I doubt they would suddenly be considered “the red one” and betrayed by the mind flayers. In a court of law, the mind flayers’ logic certainly wouldn’t have stood. However, the Red Mage didn’t really care about the inevitable legal argument. He just wanted to get close enough to the Tower of Domination.
While others debated, the Red Mage flew up to the sphere of magic that represented the collective magical energies of hundreds of spellcasters. He then used a wish spell not to destroy the tower, but rather to drain its magical energies into himself, giving him the powerup to become the dragon-god he had once been. The PCs, realizing that they were outmatched, dogpiled on the paladin who decided to charge the dragon and then teleported to safety. Whether the mind flayers were legitimately within their contract became a moot point, since the dragon-god Derrezen has no interest in keeping bargains and promptly incinerated them all.
So the good news is that the aboleth had been defeated and the day has been saved. The bad news is that something much worse has emerged. The PCs would eventually defeat Derrezen for good, with the campaign culminating in an epic mythic-level battle that left everybody dead, but that would not be for quite some time…and not until after many sacrifices along the way.