>>226068
That actually gives a pretty good idea for another way to immortality. We already know that there are ways in D&D to store your soul forever, or to store your magical abilities.
What about a mortal mage who worked his whole life, rather than going for lichdom, to store his full magical power in a vessel, knowing that he would eventually die and be brought down to hell as a larva to be turned into either manes or a lesser demon? He would find some small plane on the Abyss, and trigger the power to return to his soul the moment it, in any form, entered into that realm.
With all of his power and memories returned, he could quickly elevate his diminished form into a greater variety of demon, possibly even molding himself into a unique form, and setting himself on an accelerated pace towards becoming a Demon Prince.
Within the story, you could have something like this: Players learn early on a story about a group of many centuries earlier, who had defeated a powerful arch-mage who had summoned demons and raised armies of the dead to damage the kingdom. According to the legend, in his death, he made a prophecy that he would return - and the world would fall under his power. And over the decades which followed, the party which defeated him began to go missing.
Over the course of the campaign, his sigil and other shit begins to appear, and a cult develops under him. More than that, strange new varieties of demons begin to appear. In time, players meet with a dwarven member of the original party, the last one left alive, and learn that these are the same signs.
Further, it turns out that their party was never really that powerful - in fact, the elf makes note that the mage seemed far too weak, having been wielding magic items more powerful than his own spells - by that logic, he should have had an easy victory. Something had gone wrong. In the years that followed, he went into hiding in the dwarven lands the players find him in, as his companions began to go missing. As the story continues, the other members of the party are met as strange demon-constructs powered by gems with the trapped souls of the original heroes powering them.
The current party, by this point, is now much greater in power than the original one, and begin to deal with the mortal followers of the new Demon Prince. Further, they begin to learn of his origin. Eventually, they make plans with the king ruling over the land to lead a large force against the leader of cult - only to be betrayed, as the ranking officials of the local military are members of the cult, and sealed away in a large dungeon filled with demons and other stuff. They are forced to make their way through the darkness which was the former lair of the Demon Prince while he was still a mortal. At the end, they are pitted in combat against a fight way beyond their means against a Iron Golem powered by the soul of the now deceased dwarf from the former party, only to be aided by a group of lower-level but still competent NPCs. They reveal themselves to be followers of a Daemon-cult, who are worried about the possible imbalance caused by the forces of Chaotic Evil being so heavily bolstered, and the possible chaos within the Abyss of a new Prince.
Either with or without their help (without being significantly more difficult), players find out the king has been replaced by an illusion maintained by the new Demon Prince. They confront him, expecting to meet the cult leader - only to find the Demon Prince has already been partially summoned, with the king being a manifestation of his mortal powers while his body prepares to fully emerge from the abyss.
Players (hopefully) manage to beat him back in his current state - if they do not, he manifests fully, and it is very likely an early game over. If they do, his return is averted temporarily, giving them time to rally the defenses, as the leaders of the demon cult retreat to their fortress.
It is brought to the players attention that there is no easy way to destroy a Demon Prince - thus, they are forced to either seek sage advice, or are provided the necessary knowledge by their allies, to seek out a relic which will permanently bind a single extraplanar being to the world, limiting their powers in the process, and making it possible to destroy them without heading into the Abyss.
From there, the story can go:
> Find the relic. This involves encounter a powerful angel - who may possibly persuaded to give it over, unbinding himself, or may simply be defeated in battle and the relic taken by force. With the relic, the party may attack the enemy at the height of their power, but also defeat the Demon Prince forever within the Prime Material plane.
> Attack directly. This means that the enemy may not have their defenses fully prepared, meaning less demons summoned and so on, but the only way to defeat the enemy is by entering into the Abyss, and defeating the Demon Prince permanently there.