This 7 year long mistery has finally been solved, thanks to the player Brighid Grand Explorateur from Gentebra. There was a popular belief (myself included) that the task Mend What's Broken was needed to open Drak Grounds, but the answer lies in another task given by the Dream Butterfly. The report of this finding can be found here. There's also an interview with the player. Unfortunately, I believe it's only available in Portuguese.
To open Dark Grounds area, 20 Soothing Bad Dreams tasks must be completed. This task is also given by the Dream Butterfly but it's not necessary to get the Dream Warden outfit, hence the reason the area was so rarely open. According with the Brighid Grand Explorateur interview, theses tasks don't have to be completed in the same Server Save cycle, since he found this out by doing the task twice per day.
After that, a ladder will appear nort of the Roshamuul depot and it gives access to the Dark Grounds. The area will remain open for 15 to 25 days, depending on more tasks being completed.
There are a few steps required to complete this task:
- Get the task from the Dream Butterfly. You receive a Delicate Pan .
- - On the north-eastern of Upper Roshamuul, use the water of the Dream Sand River until you gather Dream Sand .
- - Find and use 3 special Sneeze Blossoms , located here . You will receive Strange Good Night songs .
- - Equip the Strange Good Night Songs on the shield slot and go to the eastern part of Upper Roshamuul. Use Dream Sand to soothe five Bad Dreams.
- - Report back to the Dream Butterfly and get your rewards (3x Essence of Dread) up the stairs on the Junkyard Tar Lake.
Sources:
Portal Tibia - Mistério de 7 anos solucionado!
Portal Tibia Fórum - Entrevista com Brighid Grand Explorateur
Tibia Fandom - Roshamuul Quest, Soothing Bad Dreams