"This then is the Cave of the Nadir: aswim with auroral light, filled with a deep silence that makes the Forgotten Quarter seem like Spite on market-day."
The Cave of the Nadir, accessible via the Forgotten Quarter, is bathed in irrigo. Given this Neathy color's nature of making people forget, the cave has a surprising number of residents. It's a wonderful place to survive the fall of a city. However, irrigo has drawbacks: other than memory loss, it can make those exposed to it grow bony plates over their eyes. These people eventually die, and yield the infamous Eyeless Skulls. (It is assumed that this is because the human body reflexively tries to shield itself from irrigo.)
I'd include more details about this location, but, well, I forgot.
QUEEN OF INKS (Spoiler Warning for Seeking Mr Eaten's Name!)
"I could try to tell you the way, but it can't be told, only shown. I don't recommend it. It's not safe. I've only replaced one set of dangers with another."
"That's what it's usually called. The Bazaar can't find it. There are powers the Bazaar fears - they can't find it either. This whole city is under its protection, I think - the whole Neath, perhaps - but to carry that protection with me, I had to go in deep." She sighs. "I lost a great deal."
Millicent Clathermont, alias Lilac, descended into the Nadir and sacrificed much of herself in order to be infused with irrigo. This made her immune to the deadly effects of sunlight on ordinary Neath residents, and also immune to being found because anybody who sees her will forget her once she's gone.
The part of her that she sacrificed is still in the Nadir, and it guards St. Erzulie's Candle. If the player takes the candle, Lilac insists in coming with them, noting that the Nadir has reduced her to nothing but the candle's guardian, and without it she would be nothing at all. So for the rest of the torturous journey, she accompanies the player, making parenthetical comments.