I just fixed this. So reload your save before you destroyed the blighted barricade, and instead of destroying the blight barricade before the essence doorway. If you can't do that, just see if the following works.
Look around for any blight blisters, I had a red trail leading to the upper part of the cliff face. Even though the barricade was pop-able, I hadn't destroyed that. I destroyed that, destroyed the barricade, and used the essence on the door... and the boss was there.
I had a theory about something going wrong on the quest because after poping the blight barricade, my quest marker would show up on the map as a blank objective around that area.
Found another post with the solution, it has an image here: https://forums.ea.com/discussions/dragon-age-the-veilguard-tech-issues-en/last-part-of-the-heart-of-corruption/5007222
Omg! Thank you!!!! That 100% worked for me!
