Exploring the Old Underground Complex

On Wednesday, I shared an idea that I found from 2008 about undead creatures. I’m not sure why I never used it, but the idea sits in an old email, never to be seen until just the other day. I found some more cool stuff in the email that I would love to use, too. It’s about adventuring in an underground complex that has been closed up for many, many years:

• In a dark part of the cave complex, they find a door that leads to a finished underground hold. I need to find out what creatures used to live in this hold. The hold isn't populated now. The hold is/ was not a dwarven complex. It's not Skaven. I need to find another underground creature that might have made and live in an actual underground keep.

• Now the complex is filled with undead.  The undead are not human, they are undead of whatever creature used to live there.  This could make it doubly scary, a monster and undead in one.

• There may be treasure there, as the undead keep looters away.  I'll actually have dead looters at random places, killed in ways that undead kill people.  Maybe the party could find the looter bodies before they find the undead, as clues.

• Magic items, potions, traps, etc will all be appropriately aged.  Who knows what a potion 500 years old might produce as a result if quaffed, even if the potion is labeled "Flight". 

• One of my main bad guys, or the main bad guy in my head right now, is a very old vampire.  Matter of fact, he's all desiccated and lying in torpor, shrunken in his coffin.  He isn't controlling the undead at first, because he's not conscious, but once he's awoken, he's in charge!

• Rooms will be filled with everyday items, from the time, just very old.  There could be some masterful weapons, magic items, gems, jewels, coins, etc that aren't decayed, plus an array of decayed items.  'Oh, look!  A spell book!'  The party picks it up and it crumbles in their hands.

• I can have working traps and non-working traps.  I can have broken and rusted traps.  I can have poison traps where the poison has long since expired, though the party might not figure that out, if they spot the poison coating.

• I can have locks and puzzle boxes.