It’s in a random location for your file, and it’ll sit in a chest there until you go grab it. The second way of starting this quest is by finding the beacon in one of those locations. However, she’ll need you to grab the beacon first, so you’ll receive a quest market to one of the over 150 locations that it can randomly be found. Meridia will speak to you and inform you that a defiler has begun desecrating her temple with darkness, and she wants you to deal with it.
To do it yourself, find the Statue to Meridia, which is located on Mount Kilkreath near Solitude. There are two ways to initiate this quest, although one of them is more like the quest initiating itself.
