I have had two examples, one from 5E and one from 1E. The 1E campaign wasn't very interesting by modern standards, as it was an episodic campaign with unconnected adventures. Eventually the party reached 30+ level, and I told them I'd make one last epic adventure where the party would become demi-gods or be destroyed forever (the other option was an army of level draining undead that should bring them back to more manageable levels). They managed to complete the adventure, alter the timeline (setting up 2E), and become demi-gods, wherein we ended the campaign.
In 5E I ran the equivalent of an AP using classic Greyhawk adventures. The party started in the village of Hommlet looking into bandit activity, which revealed a newly revived cult of Elemental Evil. They defeated the cult, freed the Prince of Furyondy that had been imprisoned by them, and destroyed the weakened imprisoned form of Zuggutmoy. They were rewarded for their services with a fort in the western hills, with one PC becoming a baron by marring into nobility and granted the title Watcher of the West.
Word came out that Keoland (a rival nation) was seeking out the Lost the Lost Caverns of Tsojcanth, the old lair of the witch Iggwilv located in the Yatil Mountains that supposedly contained a powerful artifact. They sought it out as well, in order to gain the prize before them, but were distracted when they found a strange forgotten temple dedicated to an unnerving deity (Tharizdun). They eventually discovered the Lost Caverns of Tsojcanth before the agents of Keoland, and defeated the vampiric daughter of Iggwilv to recover Daoud's Woundrous Lanthorn. They teleported away before Igglwiv or the rival group could catch up to them.
Diplomats from Keoland explained that they desired the Lanthorn because it is supposed to able to break any darkness, while a dome of shadow had covered the capital of Stireich, an ally and former province of Keoland. After concessions to Furyondy, the party brought the lantern to the royal court of Keoland, where they gave the Lanthorn over to the court's archmage. They were also told that the same area had been heavily attacked by giants, and the PCs agreed to fight Against the Giants to help the now decimated people of Stireich.
Afterwards they found they were part of an alliance with strange elves from deep below the surface (drow) and had dedicated themselves to the same deity from the lost shrine they discovered in the Yatil Mountains. They were able to chase these elves Into the Depths of the Earth into the Underoerth where they found the Vault of the Drow. They found that the drow were broken into factions, with most supporting the Demon Queen of Spiders (Lolth) and a small faction supporting the new god Tharizdun. They defeated the Tharizdun faction, but discovered that the Lolth faction was behind the shadow dome over the capital of Stireich.
They faced off against the priesthood of Lolth, even defeating the avatar of Lolth herself. She then taunted the party that her plans remain unchanged, as she could draw Stireirch and eventually the entire world into her Demonweb. They found a portal to the Demonweb, where they traveled along its maze only to find the archmage of Keoland had been defeated, drawn into the Demonweb, and transformed into the lowest form of demon. The party located the drow who'd taken the Lanthorn, as well as another portion of the abyssal plane that contained a strange structure.
They investigated the structure to find it was a ship that could travel through dimensional darkness to draw everything into the Demonweb. They vowed to shut it down, wherein they faced Lolth herself, first as a simulacrum, then as the demon herself. After an epic battle, Lolth was forced to abandon the Demonweb for another abyssal plane she controlled, causing the ship to collapse without her magic. They returned home via a Gate and were elevated as heroes by both Keoland, its allies, and Furyondy.
The campaign ended at 18th level, but we've kept the characters, since I have a one shot we may revisit these characters with.
