Cradle of Sulevin is a mini-location, which you can access after you find a piece of information, at Emprise du Lion about the Sulevin Blade. Here, you need to find 4 fragments of a sword that can later be crafted into a legendary sword for you, by Dagna in Skyhold. The only opponents here are skeletons and the undead, who are summoned by the light lit on the statues and are in the possession of the fragments.
The way to unlock the location: In Emprise du Lion, find the notes on the Sulevin Blade - the "Ruined Blade" quest (to find the description of this quest, see the walkthrough for the game).
Recommended experience level: In the ruins, there only are the undead and skeletons. The level that is sufficient for completing the quest without problems, is 11.
1) Unlike many other minor locations, you will be able to return to this one at any moment, using the world map. This is why, if you decide that your experience level is too low to defeat the opponents, you can retreat and come back later.
2) To collect the blade fragments, you need to use the beacon to light four statues in the ruins and kill the summoned undead.
3) In this location, there also is one type of glyph (rune) and a mosaic fragment.