Glacial Tarn is the sister dungeon, and sequel narratively, to Empyrean Forge. Back when I was designing Empyrean Forge, I took on the challenge of designing the dungeon layout to both tease future content and to build within the constraints of an area that would, eventually, double in size.
I always start by thinking about the play spaces architecturally. I value visual legibility and playable navigation above all else, but I also love spaces that have well defined character with logical, narrative details supporting their inclusion. In this case, I designed Glacial Tarn as though it was a runaway coolant system for the massive Emyprean Forge facility. The magically imbued ice water had, for centuries, been left unattended and had chilled the entire facility in nearly indestructible ice.
In order to break through the ice, players would need to find a new power source to fuel their attacks: The Empyrean Flame. Players would find flaming vents throughout the expedition that buff their attacks with fire damage strong enough to remove Ice Blockers from doorways and switches and Ice Armor from enemies. I designed this mechanic to push our status effect systems to new utilities, further opening up our designer toolbox to include more lock-and-key style gameplay features and more interesting encounter types.