Demilich

 Not gonna lie, despite being a fan of big bad stat blocks that give you a lot to work with, this one scares me. Not only is the stat block decently complex, but liches, and in turn demiliches, have been a staple of the hobby far, far before i got into it in the early 2010s. These things have legacy, in amounts i can't truly grasp.
This, funnily enough, translates pretty well into how i believe they should be treated in-game.I think most encounters should have at least a bit of context to them, lest they feel video gamey - but demiliches? Their very existence in an area shapes local myths and legends, disrupts flows of magic and fundamentally affects histories of entire races. 

The heroes of Sorya have never met a foe as ungodly persistent as Nal'Galosh. When they first thwarted his plans, he was but a cult leader who had started delving into necromancy. As he gathered power, both personal and political, they fought his endless hordes of undead - but failed to stop his ascent to lichdom. Years passed, and the heroes trained and prepared to meet their foe for the final time - but it was not to be. While the lich fell to the barbarian Ganur's axe, the huge warrior - agains't the wizard and cleric's urges - took the mighty undead mage's skull as a trophy. Just as they reached Sorya, and were passing an ancient burial ground, the skull suddenly animated, and stuck once more. The fight was long and hard, and claimed Ganur's life, but Nal'Galosh was finally laid to rest.
In the years since, the heroes have become the world's guardians, called against the greatest threats, and drank from the fountain of eternal youth. A hundred and one years after their last encounter, Nal'Galosh somehow returns. With a body of pure negative energy, and legions of shadowy creatures at his command, he wages war once more - and the heroes grow weary.

In their travels across space and time, the crew of the spaceship The Rake encounter a truly bizzare tomb world, orbiting a dying star. The world is littered with thousands, possibly even millions, of small tombs - each and every one containing a skull with bizzare crystal outgrowths. The ship's scans show that the planet is somehow dampening the skull's energy signatures - but surely at least one should be removed for closer research?

In his prime, the Three Sisters of One Tooth served the lich Parafloscus dilligently. In the years since, the once influential undead scholar retreated deeper into his labs to further his alchemical studies, which, over time, resulted in his body devolving into that of a demilich - while his mind wandered off gods know where. The three night hags, who now command the myriad monsters that infest their lord's realm, rarely directly confront intruders - instead, they rely on curiosity being a killer. Indeed, many a grave robber was enticed by a strange, fungus bearing skull laying neatly on the floor of the laboratory, only to learn that the fungal outgrowths indeed trap souls of heroes long forgotten. The hags, in turn, have developed methods of safely stealing the souls from their former master - but that means the undead is mad with spiritual hunger whenever encountered.