WHAT IS IT?
An Ossuary, to continue with the rather morbid theme of the Necromancy posting is a site that serves as the final resting place of human skeletal remains. The body is buried in a temporary grave and then moved after a certain amount of time.
Because they are not using the coffins or even keeping the skeletons together there is a great deal more space available. 
WHERE DO YOU GET IT?
In and around the churches between the 8th and 12th centuries. If you've stumbled across one somewhere else then it's good sign that something untoward has been going on. That being said the Douaumont ossuary in France contains the remains of more the 130,000 French and German soldiers that fell at the Battle of Verdun during World War 1.
USING IT IN REAL LIFE:
An Ossuary has but one purpose and that is to store bones. Though it might be a good place to hide if you were on the run from someone. Technically the Ossuary would still be storing bones they'd just be a little fleshier then usual.
You might have one if you were studying the effects of a particular item on bone structure or density and you might want to branch out into a body farm which allows people to study the effects of decomposition in corpses under various environments and situations.
As with the Douaumont ossuary it could be that the battle is over and there is no way to identify the bodies of the soldiers or anyone on either side who is able to properly claim them.
USING IT IN WRITING:
I'll stay away from the magical aspect that might be present since I pretty much covered that yesterday 
- it could be that the secret escape passage is located in the ossuary.
- It would make a delightfully spooky place for a nightly rendezvous or picnic 
- A particular religious observation might see a family spending the day amongst their ancestors where something unexpected may very well happen.
- An earthquake or similar disaster reveals a hidden ossuary that could catapult the hero into action.
- An ossuary might be disturbed by looters, who then begin to die in a variety of different ways. Is it a curse or worse? 
MIGHT ONE CONQUER THE WORLD WITH IT?