Haven – "Everything is mixed up, truth and falsehood" [Campaign 3 – Mission 2]

Here's my situation: the "Choice" and "Rebellion Alliance" quests have checkmarks next to them, meaning I've defeated all the barons and chosen the "path of tears."
I still have the "Mournful Song" quest – I need to find the artifact "mournful song" – and the "Unfinished Business" quest – I need to find the Hidden House.

Is the "Mournful Song" quest mandatory? (It's listed as a side quest and marked in blue). If so, where should I look for it? What is the Hidden House? There's a green Necropolis castle in the dungeon, but it's blocked by gates that are closed.
I'm waiting for an answer; I really want to continue playing, and I'm getting tired of wandering around this map.