Mass Grave is one of the haunted site events. You call up the dead with Astral Communion and fight off the spirits, and the reward chest holds the Soul Reaping Manual.
Come in daylight. Scan with Focus Mode, loot the note from the yellow body, then use Astral Communion on the other one. Kill the red wraith first, then the two smaller spirits, and open the chest.
The chest gives the Soul Reaping Manual. Some guides also list epic Tough Leather Boots, but that is not confirmed.
Image: Rebel Wolves / Bandai NamcoAmbrus's Cellars
Ambrus's Cellars are the court activities where you raid the hidden blood stores of the vampire Ambrus. Each cellar you clear raises his anger, which is how you unlock the duel with him.
Image: Rebel Wolves / Bandai NamcoAmbrus's Cellars: Trouble Brewing
Trouble Brewing is the Ambrus cellar hidden in a guarded house in Svartrau City. Spare the alchemist upstairs and you walk away with a Lasting Malediction Manual and a potent Razor Claw vial.
Ambrus's Cellars: Uninvited
Uninvited is the Ambrus cellar near St. Tyna's Grove, an outpost with ruins below it. It is the toughest of the three cellars because a named murohn guards the blood stock.
Image: Rebel Wolves / Bandai NamcoAmbrus's Dungeons
Ambrus's Dungeons are court activities where you break into Ambrus's prisons and decide what happens to the people inside. They count toward his anger, which unlocks the duel with him.