I already found the luminai shrine long before I got the quest. Now that stone plate thing is missing. the object ID is LostDietySteele01,,, I think. I had it in my skingrad house, but it suddenly disappeared, and it's not at the shrine of luminai either. How can I complete this quest?

In your position, I would look up the formid of the stele, enter the game, open the console, and type:

player.additem formid 1

This will add the stele to your inventory, and hopefully advance the quest.

If that does not work, you will have to figure out the quest id and use a setstage command to progress.

I can't seem to get this to work. I'm entering:

player.additem 01001e3a 1

and it keeps giving me a long message about item not found parameter something something.
I tried adding gold, and that worked. But i cant add this item. Anyone know what I'm doing wrong?

The first two parameters of the form ID need to represent the load order position of Lost Spires in your game. For example if Lost Spires is the 16th esp in your load order, then the Form ID would be 16001e3a

If you use Wrye Bash or OBMM it will be easy to retrieve the position number of the Lost Spires by viewing your load order.

Thanks a lot! I've learned something new.

So, just in case anyone else finds themselves in the same position....

I used player.additem formID 1

Then, I dropped the steele on the ground, and picked it back up. This triggered the quest completion.