Tomoe's Darkness is a mask earned by finishing New Game+. It is the one New Game+ mask Baku does not sell, so you have to clear the full second run.
Start New Game+ after the credits and play the story through to the end again. The mask unlocks on completion.
Adachi Dragon Mask
Adachi Dragon Mask is a New Game+ mask sold by Baku the Voiceless for 5 Ghost flowers. It does not exist in a first playthrough.
Adachi Half Mask
Adachi Half Mask is a mask you get by finishing "The Sister Betrayed", from Masako's tales, part 9 of 9. It is a story reward, so playing the tale is the only way to add it.
Ancient Dragon
Ancient Dragon is a New Game+ mask sold by Baku the Voiceless for 5 Ghost flowers. It does not exist in a first playthrough.
Avenging Lord
Avenging Lord is a mask waiting at a Gift Altar once you finish "The Broken Blacksmith" in Jin's Journey. Each main story milestone unlocks a different altar mask.