TLDR: Go to Kragenmoor to the Grandmsters Palace and find Grandmaster Omin Dres to begin the quest Aggressive Negotiations.
Elder Scrolls Online on consoles has an achievement/trophy called Hero of Ebonheart which requires you to complete all the in game achievements relating to the Ebonheart Pact.
As I was checking through the list of ones I completed, I noticed that for the achievement Hearts of Darkness, the middle step – Stop Tanval from unleashing the second Brother of Strife – was incomplete. When I checked the map there were no black markers normally indicative of an incomplete area, nor did I have any incomplete quests in Stonefalls.
It turns out there is short series of side quests that are easy to miss in the Kragenmoor area starting with Aggressive Negotiations. This eventually leads to To the Tormented Spire which adds the Tormented Spire as a location to your map. Complete this quest line completes the middle part of the Hearts of Darkness achievement.