I don't remember in which book it's mentioned, but it basically boils down to someone telling Harry to NOT go into the NeverNever on the Island.
I think it was a villain, actually, taunting Harry about how he had no way to escape. Maybe Nicky-Nick & The Nickleheads?
Something I vaguely remember as 'I know what lives there. You're better off letting me cut your throat.'
Definitely Mab then, Harry was still avoiding being the Winter Knight at that point, which was probably common knowledge at this point, and if the Ice Garden, then that’s where Lloyd Slate was kept, Harry’s predecessor, whom Harry’s slit the throat of. Nicky would easily guess that would be required of Harry, and knew Harry would rather slit his own throat (or as actually happened hire a sniper) than be Winter Knight and Kill an innocent human (although Lloyd Slate v Maggie, foregone contest)
A great find adding to my theory, that Demonreach comes out at Arctis Tor.
Nicky knows where it comes out and doesn’t dare use it himself. Not without using up his precious favour to Mab, perhaps and it would be a grievous insult to Winter to arrive at Arctis Tor without invitation, no Guest Right would apply and Nicky most likely would be next to Lea and Lloyd if he tried it.
Harry as Winter Knight has a right as a senior Vassal to enter Arctis Tor unless forbid like Nameless who is bound by Winter Law and can’t even try.
The Mothers are Mantles and are artificial constructs apart from the person or being they were. No doubt Hecate and the Fates poured all or part of their power into the Mantles, the beings may have retired like a certain bartender, living in the Mortal World, powered down, at a power level much less than the Queens. We know they weren’t Queens or Ladies in the last millenia. The other option was to embrace the Mother Mantle but live permanently in the Never Never, to prevent reality breaking in line with my theory of the White Gods Proscription.