It wouldn't be marked on the map if you found it earlier, though. I stumbled across it within the first few hours while playing, and it didn't register on my map until I did the quest much later. I even have a little notebook where I was keeping track of, among other things, "keystone statues" for that very reason.
I can't remember if the map gives you a red/green arrow or just reveals the location (in which case you can tell because it'll be lighter than the places you've found while exploring until you actually go there), but it's easy to miss because it's further south than you might expect.
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- It's in the southern part of the Middlerealm, ESE of Tirin Abbey and almost directly east of Erodan's Chapel.