Under what specific celestial and monthly condition was the portal to the Lost Level of Undermountain visible from the sewers?
Here's what the lore says, drawn directly from the indexed sources:
Waterdeep/Sewers > Waterdeep/Sewers > Functionality > Interior Connections The sewer network notably featured connections to the dungeon complex under Castle Waterdeep, the Dungeon of the Crypt, and in several places Undermountain. Many such links were kept operable and secret by the many guilds and shady organizations that operated in the city. Connections with Undermountain in particular featured heavily in the city's tavern lore. - Dungeon Level: This first level of Undermountain had a tight winding crawl space, the Grim Passage, that ended at a secret door high above the headless relic…
Source: Waterdeep/Sewers
Undermountain > Undermountain > Entrances and Exits The halls of Undermountain were linked to the surface world in many ways, both physical and magical. Some worked by means of one-way portals. The dungeon had a number of connections with Waterdeep's sewers which indirectly connected to other dungeons. Halaster used his network of portals to bring in monsters from all across Faerûn to restock his dungeon. Some of the notable physical links are described below: - Blue Mermaid Passage: This connected a dry well in the cellar of the Blue Mermaid tavern to Skullport. - Castle Corkscrew: This…
Source: Undermountain
Yawning Portal > Yawning Portal > Entries to Undermountain > The Wet Well The second, or "wet", well was a normal well in the sense that water could be pulled up from it. It was located in a room near the rear of the inn and was primarily the inn's source of washing water. Although rarely used for anything more than water, Durnan kept the door barred, with a glittering sword above the door, a torch in a wall bracket outside, and rope, torches, and a tinder box ready on a table inside. This well was no more than a hole in the floor, usually kept covered with a wooden hatch embedded with the…
Source: Yawning Portal
Skullport > Skullport Skullport, also known as the Port of Shadows, was a subterranean city located far below Waterdeep, within the 3 level of Undermountain known as the Sargauth Level. While rumors of this subterranean domain of villainy persisted throughout the surface world, few of its inhabitants had any knowledge of any points of entry. "The Port of Shadow is unlike any port above or below the surface of Abeir-Toril. Not surprisingly, it is much like the Underhalls that surround it: difficult to quantify, enigmatic, dangerous and yet alluring, all at the same time." - Alphraxis of…
Source: Skullport