Shared Greyhawk/Mystara Elements
by HåvardI have found that the following elements exist in both Greyhawk and Mystara:
Deities:
Orcus
Demogorgon
Yeenoghu
Brindobris (Brindorhin?)
Garel Glittergold
Kurtulmak
Vaprak (aka Jammudaru)
Ilneval (aka Karaash)
Maglubiyet (aka Wogar)
NPCs
Warduke
Peralay/Melf
Egg of Coot
Locations:
Keep on the Borderlands
Isle of Dread
Blackmoor
Duchy of Ten/Duchy of Tehn
The modules B1-4 were published before the Known World was concieved, though Gaz1 provides suggested locations for them in Mystara. I suppose these could be considered both Greyhawk and Mystara sources?