Going from memory, this will be another vague description until someone makes a better solution, so bear with me.
*** Spoiler - click to reveal ***
In the crossroads, you will have to get 4 pieces of a set, each inside a chest guarded by demons.
1st, when first arriving at the crossroads, instead of following the blood trail, go around the the right around a wall to find another eluvian, there's a chest at the end- activate it to trigger the fight. win and take the loot.
2nd is after you return to the crossroads and you go down the stairs that weren't there before. Instead of following the qunari, go around behind to the right and find a red bulb, activate it and follow the stairs that form up to another eluvian. there's a piece of the set in here, follow the same steps once inside as the first.
3rd is in the same run, back at the eluvian you're supposed to go through for the quest at the end of the stairs, instead go to the left up around the rocks to find another eluvian. rinse and repeat.
4th, is once you return yet another time to the crossroads, and go up the stairs from the second area you got to previously- this new area should have bookshelves and stuff on it. From here, if you ignore the mission quest eluvian, there's another red bulb nearby off the path to activate- again more stairs appear to the last eluvian and more demons. After you pick this up it should pop. Some of the demons put up a good fight with a rage demon and two high level despairs.
Also to note, this should be possible to get with backtracking so long as you haven't gone through the final eluvian section with the keystone straight ahead of the entrance from the winter palace.
Good luck!