Is Erelhei-Cinlu in Faerûn?

Question

I have a major campaign extension looking to take characters from 1st to 20th level, starting with linking Lost Mine of Phandelver to (via various add-ons to LMoP itself) adapted versions of D1, D2, D3, Dungeon of the Mad Mage, Expedition to the Demonweb Pits, and more.

I intend to have the PCs travel through the Underdark from Wave Echo Cave (I've created extra levels below it) to Erelhei-Cinlu, but am unsure if Erelhei-Cinlu is actually in Faerûn or if I'll have to drop in a portal to Oerth somewhere along the way? Advice?

I'm using mostly 5e with bits and pieces of 3.5e and some stuff of my own creation to help flesh stuff out...

Answer 1

Erelhei-Cinlu is on Oerth

There is no officially published conversion to 5e from WotC for the D-Series modules, but for several other modules 5e has issued recommendations on relocating things from other game worlds to the Forgotten Realms.

Most relevant here are the "Against the Giants" modules (G1-G3, G-Series) re-published in Tales from the Yawning Portal. These adventures like the D-Series with the city of Erelhei-Cinlu you are interested in all originated in Gary Gygax' home Greyhawk campaign, the original campaign the led to the publication of Greyhawk as a setting. The D-series continued the storyline of the G-Series, and the intro to D3 (p. 2) states:

As a member of a bold party of adventurers, you and your associates have trekked far into what seems to be a whole underworld of subterranean tunnels—arteries connecting endless caves and caverns which honeycomb the foundations of the lands beneath the sun. From the entrance to this maze found on the lowest level of the HALL OF THE FIRE GIANT KING (DUNGEON MODULE G1-2-3), your expedition has dogged the heels of the Dark Elves

So if you locate the Hall of the Fire Giant King, you know the entry point to the underdark for the D-series. Based on the guidance from Tales for the Forgotten Realms the location of the Hall is one of these:

In Faerun, the giants could be raiding the Sword Coast after emerging from the Sword Mountains or the Sunset Mountains or the Spine of the World. They might be preparing to move on Damara from out of the Galena Mountains, or to descend upon the Dalelands and Cormyr from the Stormhorns or the Thunder Peaks.

Either of those would be a recommended starting point for the underdark hex map provided in the D-series modules. This hex map shows the location of Erelhei-Cinlu relative to that entrance point. As the overall extension of that map is only about 50-60 miles, it would be pretty local to whatever mountain range you choose, compared to the size of the Sword Coast map that covers over 1,500 miles.

The adventure locations are introduced like this in Tales:

PLACING THE ADVENTURES
The preferred habitats of hill giants, frost giants, and fire giants exist in virtually every D&D setting, so it's not too difficult to find locations

Both from this and the flexibility of suggestions where you could place it, it seems all you really need is a mountain range with human lands to be raided nearby for your starting point, and you should be fine to put it whereever it makes sense for you.

A major drow city in Fearun would be Menzoberranzan, which is described in the module Out of the Abyss, along with a description of the Fearunian underdark at large, which may be of use.

One piece of advice I have is that the D-series is targeted originally at character levels 9 and up. If you come straight from Wave Echo Cave, as your party should be at best level 5 after Mines of Phandelver, you'll have to bridge the gap up to there if you want to convert to the levels.

Answer 2

If you want it to be

It’s your campaign world, put in anything you like

The original location of Erelhei-Cinlu was … nowhere in particular. It first appears in the D-series modules which were originally untethered from any campaign settings because there weren’t any published campaign settings at the time.

The original printings of the G-D-Q modules make no reference to anywhere outside of themselves. Where they were in any particular DM’s world was up to the DM. That is, if the DM even had a world.

In the 1983 boxed set of the World of Greyhawk, suggestions were made on where these (and other modules) might be located. From memory, there might have been more than one. Ultimately consensus settled on the western regions of Keoland.

AFAIK, there is no consensus on whether this city exists in Faerûn. But if it’s there in your Faerûn, that’s fine. No one, not me, not WotC, not Ed Greenwood, and certainly not “canon”, can tell you what to do with your Faerûn.