This question might be better suited for World Building SE, but because of how specific it is, and how it is meant to be shown through gameplay, I thought that it would be more on topic here.
An immortal child's playground
Imagine, if you will, a city that swallowed by the earth nearly 2000 years ago, and its ruined remains 200m below ground. In a series of underground caves and uncollapsed buildings, a well of necromantic power.
The destruction of the city has left many corpses in the caves, and the well passively animates and preserves them. However, without any mind controlling them, they are inactive.
At some point in the following years, one of the animated bodies, a teenager body, mysteriously got a soul. A newly born soul. They had no memories, no experience, no knowledge. But they have innate (sorcerer-like) powers, and managed to take puppeteer-like control over the other animated corpses.
For the following ~1800 years, they lived there, alone, unaware of the surface and other sentient beings, unaging and unchallenged. But a couple weeks ago, a wizard-scientist landed in the caves after a failed teleportation experiment. The undead teen used his mental powers to extract knowledge from his mind, about the surface world. Since then, he has done some expeditions to the surface bur remains a mystery.
He started a chain of events that got my players' party on the case, looking for the missing wizard as well as other persons that the teen kidnapped. They don't know about his involvement in this. They only met him once when they stopped him from syphoning a source of faery magic, and don't know anything else about him.
The point, finally
The party is about to reach the underground caves, and will explore them as a (short !) dungeon. I would like for them to discover elements of the teen's backstory through environmental storytelling, but I find it quite difficult to figure out what such a being would do for centuries, and what traces would remain that the adventurers could use to piece together what's going on.
Keep in mind :
- The teen had never met another soul since "birth" 1800 years ago
- The teen doesn't eat, drink, breathe, produce waste or have sexual impulses (or the related body functions)
- This is about his backstory before figuring out the existence of the surface
- Information should be found over maybe three locations in the cave
Do you have any suggestions and tricks that you used to convey backstory ? Sorry for the long post, there's a lot of context to this one...