Heart of corruption help

[rackjite]

The three locks on the door missing one. Have the red one. Have the green one, do not have the one on the right. Which essense dude am I missing. Probable forgot to pick it up. Where to go back and look?

[Toad]

Blue is found on Crossroad island with all the darkspawn. When you first use the boat it will take you to an aera called "Heights of Athim", If you're unable to progress to "Fall of the Protector" which is the name of the mob, you'll need to eliminate the corresponding Darkspawn Champion. I think the champion was located in the Hossberg Wetlands