Living Greyhawk Gazetteer and From the Ashes on Philidor the Blue:
Does it get more obscure than Philidor? Philidor the Blue was created in the From the Ashes era by Carl Sargent as a mysterious Gandalfian benefactor for the troubled forces of good, in particular the Vesve Forest in their fight against the demigod Iuz. He evidently appeared in the Greyhawk City one year BEFORE the Wars ended and despite never having lived there before, or belonging to the Wizards Guild, or being an ambassador, he has a house there! Phil appears as a blue human in Greyhawk, or a blue elf in the Vesve. I wouldn't doubt he'd appear as a blue dwarf if he visited Irongate. He warns Good folk of impending disasters and has set up floating globes to monitor the forest boundaries. Sargent teases in FtA that he is an enigma yet at the end of his entry tells DMs to keep him a mystery, maybe we'll tell you more about him later...
Of course, that doesn't happen. Fan sentiment was rather bad for this character and later authors swept him under the rug despite hints he had a long-term plan in mind in Marklands. The LGG mentions him twice, one that he hasn't been seen much since the Great North Crusade, but his agents and items still operate in the Vesve, and two, the last claimed sighting of Philidor was at the entrance to the Valley of the Mage. This throw-away rumor completely disentangles the wizard from all his previous lore and implies he is either allied with the Mage of the Valley or seeks to replace him. Whatever the case, Phil is gone. Everyone has their theories on who or what Philidor was, but mine is Carl Sargent was inspired by a famous French chess master, who has a defensive opening named after him. How apropos. Check it out.
Here is a bit of obscure lore. In the fine accessory written by Sean K Reynolds there is extensive, yet undeveloped lore on the southern lands of the Hepmonaland continent. A particular nation of the Touv called Byanbo references a people called the Tall Walkers. There are only two references, and one is a bit misleading.
"The Scarlet Brotherhood landed once at the city port (Byanbo) and offered gifts to the king; a small group remained behind to study the language and advise the prince. Reports surface from time to time of unusual ships on Byanbo's shores piloted by beings the locals call "The Tall Walkers".
Reading this from start to finish can almost imply the Suel blooded visitors are the ones with strange ships called the Tall Walkers. An earlier section of TBS on the Touv says they are typically shorter than other cultures of the Flanaess. So indeed, the Brotherhood could be "tall walkers" here, but then the second references clouds things:
(Yano Desert) "...an oddly constructed ruin near the hills is said to have been built by an ancient race of people that predate the Touv, possibly the ones the people of Byanbo call "The Tall Walkers".
Here it suggests the Tall Walkers were around southern Hepmonaland a long time before the Brotherhood ever visited. Whoever this race was obviously died out, or move on? There are no giants on the encounter charts of TSB except Fire Giants in the Ino Hills near Byanbo. Perhaps a connection? Or most likely the local Touv are greatly mistaken and have conflated two stories about beings who are generally taller than them. Part of me wishes the Tall Walkers are indeed a new ancient culture to throw into our Greyhawk lore. That's up to us fans!
The Silent Ones of Keoland were written for LGJ #4 by Gary Holian. One of the most intriguing characters to come out of this article is Lotakal the Caretaker, a Bard 10/ Silent One 5. Lotakal is woman who graces the cover of the issue, flanked by strange fey looking creatures. In fact, her unique role in the Silent Ones is to maintain an orphanage for extra-planar creatures and created homonculi called the Wretchery. Mind you they are orphaned familiars because the Silent Ones probably kill wizards they don't like. Kind-hearted Lotakal is different however, she knows what she is doing when it comes to forest and sylvan lore, having spent six years among the fey of the Dreadwood as a child. As an adult Silent One, the Caretaker is such an interesting NPC for players to meet and her Wretchery seems like a whimsically fun place to visit on an interlude through Keoland.
Greyhawk Players Guide on The Wandering Caravan
The 2E Players Guide by Anne Brown is an underrated accessory. If you don't have this book, get it, because its full of neat lore tidbits and Greyhawk observations you won't find elsewhere. Anne knows her stuff. One item in this book is an utter mystery and as far as I know it only appears in this book. The ghostly Wandering Caravan is a great hook for a DM to develop:
"Several times a year, a phantom caravan of merchants and guards appears on the roads circling the Nyr Dyv. The caravan - three wagons with blue-and-black-striped awnings (each pulled by four horses), six peripheral riders and horses, six teamsters and six guards - has appeared to assist stranded travelers, fight off bandit attacks on other caravans, and spoil ambushes that might cause loss of life or property. No one is certain who the members of the caravan are or when their appearances began. Some witnesses profess the entourage to be entirely ghostly, while others insist that the caravan's members are in a weird form of stasis, victims of a curse that forces them to ride the roads around the Nyr Dyv until they fulfill some goal or correct and old misdeed. Their intentions seem to be honorable, and they can be recognized by the complete lack of sound made by the horses and wagons."
So yes, the Wandering Caravan is beneficial though spooky. It sounds like they can be interacted with at times, or others they are just incorporeal. Where are they from originally? Are they related to the Rhennee (the Attloi)? This is quite a special encounter and it's in a high traffic area! They could be going through peaceful Urnst land or even through war torn Bandit Lands. It's all for DMs to expand upon.
Anything Planescape can usually be counted as lore for any setting, but one NPC from Sigil in particular has a strong Greyhawk connection. Duke Rowan Darkwood is the Factol of the Fated faction (R19/P20) and he is considered one of the two most influential residents of Sigil. The Fated (also known as the Takers and the Heartless) believe that the multiverse belongs to those who can hold it. Each makes their own fate, everyone has potential to be great, but it doesn't mean it will happen. His background certainly lends to his destined fame as he claims to be a former hero of Vaasa in the Forgotten Realms, but he is in truth from Oerth! Duke Rowan was the third son a minor noble (it doesn't say where) so with no prospects he turned to being a ranger. He ran afoul of a major devil after a "magical mishap" with a deck of cards (hmmm). Rowan was missing for ten years before he returned to Oerth, and by then everything had changed. He had got a taste of the outerplanes however, and so he followed his destiny on went on more adventures to Faerun and beyond, eventually ending up in the Sigil.
Duke Rowan may not be very significant in Greyhawk history, but he certainly would be remembered in ranger circles, if not in the annals of some noble family tree. I'd like to think he was from Keoland given its ruler is also a ranger, plus Rowan carries a giant slaying sword which he perhaps acquired back in the day fighting in the Crystalmists. Greyhawk adventurers who journey to Sigil would certainly be shocked to see one of its rulers is familiar with their world. It's good to see Mordenkainen isn't the only high-level plane-hopping adventurer from Oerth!
Anyone who owns Mordenkainen's Fantastic Adventure and the Maure Castle sequel in Dungeon #112 will recognize the covers with archmage Mordy on one and the evil Eli Tomorast on the other respectively. What they have in common is the chamber that they both stand in is part of the dungeon! In MFA, the Chapel of Kerzit is merely a non-descript chapel with a granite altar for sacrifices. Dungeon #112 expands on the Chapel giving it the gravitas it deserves for being the backdrop of both adventures (spoilers ahead).