How I resurrected an NPC ghost

Any sim, including NPCs once they die will automatically be placed in the Hidden Household section of the game. This will allow them to still be part of the family tree, and interact as sim ghosts are allowed. Now heading off to the Netherworld, there is no coming back from there, and they will be deleted from the game. This can happen two ways, one will allow a player to send them off, by clicking on the tombstone of the dead sim, and the other is after a certain period of time has passed. If your sim knows the ghost, you will get a pop-up letting you know that ghost will be heading off to the Netherworld. If you don't want that ghost to permanently leave, you have the option to click on the tombstone to stop this from happening. I don't recall the number of days the game will cause the ghost to leave for the netherworld, but I do remember it happening in my game once, and there was quite a few number of sim days that passed by first.