The Ice-Throat Sump

A flooded sub-gallery off the Karth Deep's meltwater system — a sunken Imperium shrine the river backed up and filled, swimmable under a low ice roof, with air-pockets between frozen-over chambers.

The Meltwardens ward the ice-throat above, not this. The Sump is the one part of the Deep that's unguarded, because it's the part that kills on its own clock — cold enough in the water to numb a swimmer in minutes, Karth Leeches in the dark, and a low ice roof between you and the air. The only dry way past the warded throat to the lower Deep runs through here.