The "Dark Grounds" is opened when the task "Soothing Bad Dreams" (Roshamuul Quest) is performed twenty times.
I've had the opportunity to hunt in this location, although there are 3 Sight of Surrenders, it's a great option for rapid respaw.
1. For Sight of Surrender's bestiary is the ideal place.
2. To do the achievement "Never Surrender" (kill 100 Sight of Surrenders) is a great spot too.
3. To drop the "String of Mending" to forge your Unstable Ring of Ending is a great option too.
4. This task can be accumulated, for example: it can be done 10 times in one day and 10 times in the next day. (According to information from youtuber Itexo)
5. This place can be open from 15 to 25 days, if the players continue doing the tasks the time will be extended (According to information from youtuber Itexo)