This is a campaign I started many years ago, but never got around to finishing, despite the fact that it was only going to consist of roughly ten adventures. I started it after my first experiences playing Advanced Dungeons and Dragons, Second Edition, and I utilized many of its rules and classes in this campaign, though the setting was my own. This realm was a land largely ruled by the peaceful Atoxyl Kingdom, which had gained power in a great war hundreds of years ago by forming an alliance of races to defeat the Chaban. This war involved the use of great magic and threatened the very existence of civilization itself. The victory of the Atoxyl Kingdom allowed to hold their alliance of races together, despite the enmity between some of them, such as that between elves and dwarves.
The player characters were a female gray elf mage, a halfling scout (a type of rogue), and a dwarven champion (a type of warrior priest). In this first adventure around their home town, which was roughly between a large town and a minor city in size, the scout will have found a +2 dagger coated with a strange dried green sludge in recently discovered underground ruins. He would later learn that the green sludge was an ancient poison with no known antidote; even magic was unable to stop or slow its deadly effects. The scout had also seen that there were more to the ruins, but a cave-in had prevented him from exploring further. He needed to find another way to the unexplored portion.
The mage had discovered that many people around the town were disappearing mysteriously, and had helped to prevent another strange kidnapping with the help of a nearby paladin. The paladin would become a NPC party member at that point. The champion would uncover another piece of this puzzle when he went to investigate the disappearance of his cousin. He would find his cousin dead in a cell in a nearby cave with a still living human boy. The boy would reveal that his cousin was killed by knights with strange powers; they seemed to steal life force by touching people. This is how the champion's cousin was killed, and this resembled the powers of the dark knights of the past. (Dark Knights were basically an anti-paladin class I created for this campaign.)
When these characters finally met, they combined what they had learned and came to the natural conclusion: evils of the past were returning and a dangerous poison may be hidden in the nearby ruins. It seemed only prudent to explore the rest of those ruins and see if more of this poison was there. With the paladin, they left and managed to find a way to the unexplored portion of these ruins. They encountered a low-to-medium-level dark knight here, and much to my disappointment as gamemaster, she was quickly finished off by the party despite zombie allies. Thus, they were able to find a storage room of leaves laced with this poison, and the champion used his stoneburst spell to bury this horde. This would not be the end of the adventure, though, as the stoneburst spell also starting collapsing the ruins themselves. They had to rush out to an exit they hadn't seen before with the ruins collapsing behind them.
This is where things almost went horribly wrong. Balance can be a tricky thing for gamemasters, especially since I was introducing the party to the primary villains for the first time. It had to be a survivable but unwinnable encounter; the idea was they would only have to survive three rounds. One of their enemies was a higher-level dark knight than the previous one, and the other was a conjurer. After three rounds of combat, the villians, unimpressed, would depart after the conjurer summoned some low-level monsters to finish them off.
Alas, they nearly didn't survive the first three rounds, despite the generous experience awards I had been giving away and the high attribute values of the characters. The first spell used against the party was summon swarm, bringing in a massive swarm of insects flying through their formation, making it nearly impossible for them to do anything. Failing the saving throws, they were unable to act, though this did protect them from the dark knight. The second spell used against them was burning hands, I believe, and due to the level of this mage, this knocked most of the player characters' HP very low. Some managed to escape the swarm, and the dark knight stepped forward to prevent them from attacking the mage. Their attacks against him missed, and he attacked as well.
The champion was one of the few still having enough HP to bear an attack, so I had the mage direct his acid arrow at him. The spell hit. Despite being the strongest and toughest character in the party, he was at 2 HP after another attack by the dark knight. I had barely managed to keep them alive through three rounds of combat against these characters; I had very nearly killed them. The fire beetles the mage summoned before leaving came pretty close as well. In fact, the only reason the champion didn't fall below 1 HP is because I forgot to apply two rounds of acid damage. I had remembered the first and third round, but forgot to apply the second and fourth round of acid damage. It inflicted 2d4 damage, so at 2 HP, either round of damage would have left him incapicated. (I was using the rule that -10 HP was the point of death; 0 HP merely left one unconscious.)
I doubt the party would have defeated the fire beetles without him, though. The mage, due to extraordinarily bad luck, never managed to land an attack on the fire beetles. This was despite the +4 quarterstaff I had allowed her to find, and she was taking bad hits despite the +4 cloak of protection I had also left for her to find. I believe she ran out of healing potions during this fight and she already used up her spells for the day. The scout's luck wasn't much better, and the NPC paladin wasn't doing very well either. I couldn't seem to roll any higher than single digits for him. It was up to the champion with his twin axes to finish off most of the fire beetles so they could come away intact, and he only stayed on his feet because I forgot to apply some damage. Thus, disaster was only diverted by a gamemaster mistake in the player's favor, though the disaster nearly came about because of a bad estimate of what they could handle. It was the closest I'd ever come to killing the player characters before, and technically, by the rules, I would have.