Looked for a "dead" god/realm in the Forgotten Realms setting

[Yakmala]

I'm running a homebrew campaign set in the Forgotten Realms in the area of the Silver Marches / High Forest and I've got a segment coming up where I want the party to uncover a long lost portal to a "dead" realm. This should be a realm that belonged to a god that either was killed, lost their portfolio or lost all their followers and is now lost and mostly abandoned.

Any good ideas what god this might be? It does not have to be from 5th edition, any god from the long history of Forgotten Realms will do.

[Limited Gish]

Gargauth. Relevant to Decent into Avernus.

[Rafaelfras]

There is a suplement called dead gods if you want to dive deeper in that subject

https://forgottenrealms.fandom.com/wiki/Dead_Gods

[Amnestic]

I'm running a homebrew campaign set in the Forgotten Realms in the area of the Silver Marches / High Forest and I've got a segment coming up where I want the party to uncover a long lost portal to a "dead" realm. This should be a realm that belonged to a god that either was killed, lost their portfolio or lost all their followers and is now lost and mostly abandoned.

Any good ideas what god this might be? It does not have to be from 5th edition, any god from the long history of Forgotten Realms will do.

I think most of the FR gods came back in 5e, but some who died previously are: Bhaal (Murder), Amaunator (Sun), Myrkul (Death), Mystra (Magic), Deneir (Knowledge), Helm (Protection), and Mask (Thieves).

[GentlemanVoodoo]

Look at the FR book lost empires of faerun. Plenty of choices there.

[Asmotherion]

Look into the lore on Atropals and Atropus "the world born dead". It will probably give you good inspiration.

[Pex]

The Babylonian Pantheon. They used to exist early in D&D history worshiped in Unther.

[Zevox]

A few dead gods of the Realms who weren't brought back in 5E (to my knowledge):

Auppenser - God of Reason, Enlightenment, and Psionics. Not quite completely dead, but his worship dwindled to near nothing after the destruction of Jhaamdath, the only nation in the world's history that was ruled by Psions, so he's essentially in a coma.

The Untheric Pantheon - Literally the real-world Babylonian Pantheon (imported to the Realms the same way the Mulhorandi/Egyptian one was), lead by Gilgeam. Most were killed in a war with the Orcish gods, another died in the Time of Troubles, some left Faerûn deliberately as the pantheon was falling apart, and Gilgeam himself was finally killed by Tiamat, leaving her the sole survivor.

Ibrandul - An Underdark god of darkness, stealth, and underground places, once worshipped in Calimshan and popular with adventurers. Killed by Shar during the Time of Troubles.

Moander - God of rot and decay, notably responsible for splitting the original Goddess of Luck, Tyche, into the twin goddesses Tymora and Beshaba. Killed by a Bard named Finder Wyvernspur during the Time of Troubles, who then ascended to godhood himself.

Ulutiu and Othea - Ulutiu was a Giant sea god who went into exile after his affair with Othea, the Giant mother-goddess, was discovered by Annam, Othea's husband and chief god of the Giants. Othea was killed by one of her sons, and Ulutiu voluntarily entered an eternal slumber beside her remains in the Astral Plane. Ulutiu would only leave his slumber if Othea were brought back to life.