Delves: The Waterworks

PSA: You need to stomp on corpses to collect the lost gear

Item
Stomping Shoes
When entering this Delve and getting the Suffocating Fumes debuff, you will need to speak with Foreman Bruknar in the same room as Brann/Supplies.
Once you have spoken to Bruknar, the chest by him will be able to be looted containing these shoes.

The nearby Air Purifier can be activated by using the Stomping Shoes.

This quest is not available unless you have completed Delve into the Earth on that character. Progress here is not warbound.

Because Delve into the Earth is part of the Isle of Dorn campaign, you'll need to at least start that campaign on any adventure mode characters you want to run through this questline.

And it is a questline - you won't be able to do Delves: Nightfall Sanctum in Hallowfall without having completed this Waterworks quest.... I suspect there'll be a fourth in Azj-Kahet, but I haven't got that far yet ;)