In a letter to Clark Ashton Smith, H.P. Lovecraft makes an offhand reference (like he often does in his letters, as well as fiction) to a symbol he calls the "Seal of N'gah". He also mentions the Elder Sign in the same sentence. Here is an excerpt of that letter which contains the reference:
"Again thanking you in Tsathoggua’s name for the recent shipment, & hoping to see more items from your pen ere long, I append the Elder Sign & the Seal of N’gah, given in the Dark Cycle of Y’hu."
I have done quite a bit of looking around for more information on this particular reference, and I am surprised just how little there is out there concerning the Seal of N'gah. It seems to be one of the very few pieces of Lovecraftian lore that has been ignored. Especially considering how much coverage the Elder Sign has gotten. Strange, given that both were referenced together in the same letter.
Looking in The Cthulhu Mythos Encyclopedia there isn't an entry for "Seal of N'gah", but there is an entry for "N'gah-Kthun", which is listed as a, "pre-human city in which the high temple of the Elder god Ulthar was built." This comes from "The Whisperer In the Darkness", written by Lovecraft in 1930. Still the reference is vague and may not even be related. But that seems to be the way Lovecraft wanted things.
Anyway, for the sake of documentation, and the fact that I could not find an image of this anywhere I seemed to look on the 'net, here is a scan of the Seal of N'gah:
Elder Sign & Seal of N'gah, illustrated by H.P. Lovecraft
Something that is instantly noticeable is just how crude the drawing of the Seal is. Even the Elder Sign has a certain flare and a cool mystique to it. The image Lovecraft drew of the Seal of N'gah looks hastily drawn and uninspired. This must explain why there is not a whole lot of discussion to be found on the subject. There is really nothing very cool about it at all.
If by chance there is more information on this than what I found please let me know. I would love to learn more. Special thanks goes out to Andrea Bonazzi for dragging out his copy of Selected Letters vol. III, scanning and emailing the above image to me.