Source: Into the Darklands, pg(s). 40
Harrowspire is a drow fortress located deep below the nation of Ustalav in the Darklands realm of Sekamina. Here, drow candidates for the Fighting Societies undergo grueling training to become some of the most elite warriors of their race. This training is so intense, that a majority of drow die before they can complete their education.
Harrowspire is built into a massive hollowed-out stalagmite that rests on the bottom of an extensive cavern. Entrances to the complex all exist at least 300 feet above the floor of the cave, as the ground is covered by a massive green slime colony. Unwelcome guests and failed trainees are simply dropped into this deadly lake.
Unbeknownst to most other inhabitants of the Darklands, Harrowspire actually holds upwards of 10,000 drow at any one time, along with massive caches of weapons and provisions. The fact that this is not widely known is a testament to the drow's ability to keep prying eyes from coming to close.
Harrowspire is led by an unusually deliberative and disciplined male drow named Valinqui Rasiurein, who is waiting for one of the feuding noble houses of Zirnakaynin to emerge as a clear victor so that he may throw the support of his school behind them.