Erelhei-Cinlu

Gates to the city-state.

Erelhei-Cinlu is the slum of the drow, the first Drow City in the lore. It is often confused with a "capital city" but that's not how drow roll.

Erelhei-Cinlu was introduced in D3: Vault of the Drow, underhill from the nobles' estates. Drow, as (very) Chaotic, don't meet in a central location. To the extent they share an established site, that'd be the Fane of Lolth in an offshoot cavern. And when the PCs reach the Vault, currently the drow don't share even that as the Tormtor and Eilservs houses are following a deity of elemental-evil.

The slum itself hosts 8000-9000 drow, maybe 17000 half-drow and servitors, and 1000 visitors and riffraff. Fiends and freewilled undead have the run of the place (but not the Lawful devils). If you like S&M dungeons and hardcore drugs, this is where you procure them. On the rooftops are, of course, spiders. Down in the sewers even the author didn't want to speculate.

In an irony, the drow society is so dysfunctional and the slum so wretched, that some drow dissidents therein - the Rakes - are even turning to good. The dissident movement is led by one Nilonim who, unfortunately, is now in durance vile over at the Fane.

There's a followup in Monte Cook's Dead Gods module for Planescape. Nilonim has died by then. This module retcons Nilonim into a homosexual because of course it does.

Legacy

What didn't Erelhei-Cinlu inspire, would be an equal question. This slum brought out the drow at their evil-elf worst. The most-famous successor to E-C and to the Vault generally was Salvatore's Menzobarranzan. Elsewhere there's Commorragh.

A somewhat more functional, though still hideously evil, take on Erelhei-Cinlu appeared in the Nentir Vale setting as the iconic drow city in the 4th edition Underdark splatbook.