Haunted sites are places marked on the map where spirits linger. There are four, they only work by day, and each one ends in a fight against the spirits you call up.
Use Focus Mode to find a body outlined in yellow and loot its note. Then use Astral Communion on the second body. That summons a group of spirits. Kill them all to clear the site.
Since they need daylight, pair them with other day jobs. Shapeshift or Mercurial Fervor helps you cross the map faster between them.
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.