Lethchauntos's Library (the Lost Library)

An elegant subterranean library carved by the orc arcanist Lethchauntos beneath Baldur's Gate, featuring a rotunda of shelves, summoning circles, a forge, private chambers, and a ledge overlooking a chasm leading toward the Underdark's Shrieking Abyss. It housed the Nathlum and contained traps, summoned spirits, and experimental magic objects.

A secret subterranean library carved by the orc arcanist Lethchauntos beneath Baldur's Gate, featuring a rotunda of floor-to-ceiling shelves, curated chambers, barracks, a natural cavern opening to a chasm, a forge, private quarters, and a gallery. It housed dangerous curios, experimental bindings (including Quillbit's prison), and the Nathlum, and it connects to a deep Underdark chasm called the Shrieking Abyss.

(first dungeon)
Waiting for you is a woman who looks like she belongs anywhere money moves. Her posture is calm, her expression composed, and her clothing is expensive without being loud about it. A signet ring glints when she gestures toward the seats.

“Sit,” she says. Her voice is controlled—polite in the way a knife is polite. “You’re here because I need people who can move through Baldur’s Gate without the city’s… delicate sensibilities getting in the way.”
She sets a slim folio on the table as if it weighs more than steel.

“There is a sealed ruin beneath the city—an old hidden library carved by an orc arcanist named Lethchauntos. One of his vaults contains the Nathlum, a book of arcane lore. Among other things, it details the making of rare poisons… and their cures.”
Her eyes narrow just a fraction.

“A rival has gotten there first. Maelra Szandru—an ambitious wizard with more hunger than caution. She intends to loot the library and sell what she finds to buyers in this city who should never be allowed near that kind of knowledge.”
She leans in. “Recover the Nathlum. Bring it to me. Do not let Maelra leave with it.”