Arach-Tinilith is a cathedral in the drow city of Menzoberranzan designed specifically for the training of Priestesses of Lolth. It makes up the largest and most powerful branch of the Academy, followed by Sorcere and lastly by Melee-Magthere.
Arach-Tinilith is the second largest cathedral in the city, only bested by House Baenre's own chapel.
Priestesses of Lolth
All female, the Priestesses of Lolth go to Arach-Tinilith after being trained by her house in the basics of worship of the Spider Queen. This usually happens around the age of 16, and they are to spend 50 years studying under the tutelage of the masters there.
Priestesses help fulfill the duty of Arach-Tinilith and the Academy by enacting healing spells, serving as part of the guard of Menzoberranzan, and praying to Lolth to keep the city holy.
As with Menzoberranzan and the Academy, little is known about Arach-Tinilith's origins except that it is ancient, probably starting soon after Menzoberranzan was formed.
What is known, or at least speculated, is that Arach-Tinilith was formed with the rest of the Academy as a sort of general civil-service for Menzoberranzan. Since the individual houses rarely do anything that does not in some way benefit them, it is commonly thought that a group of long-dead matron mothers banded together to form the Academy as a city guard, city executioner, and training center for all future soldiers, wizards, and clerics.