The tombstones eventually release the spirit to the afterworld and lose all the information that was on them. I keep the headstones of my legacy sims on the property and I have several generations and I noticed this early on so I added the important information that I wanted to remember in an epitaph in my newer generations, before that happened again. When the ghosts were added I had six tombstones on my property. Three of them still had the spirit associated with them so I had three ghosts running around right away. I didn't want to play them because that seems to be against the spirit or possibly actually against the rules of the legacy challenge, so I just put up with them for a while. Sometimes my sims could interact with them and sometimes not... but they randomly broke things and scared my sims. The scaring, I would be okay with some... having every toilet and shower in the house repeatedly broken was irritating. So I had a sim go to the tombstone and release the spirits and now the plumbing is functional again.