The VOIDRUNNER'S CODEX is coming! Explore new worlds, fight oppressive empires, fend off fearsome aliens, and wield deadly psionics with this comprehensive boxed set expansion for 5E and A5E!
You might consider it the supplement that "completes" OD&D. (vs. Blackmoor and Eldritch Wizardry, which already start to pull it in strange wacky directions).
The thief, and the first RPG skill system (and first % skill system at that) is there, as is the paladin, the first subclass. Variable weapon damage (daggers do d4) also debuts. It also contains many iconic spells, magic items, and monsters.
On the other hand, not the most user friendly document.
Note that the Greyhawk supplement...does not actually say much about castle Greyhawk (and EGG did not really have an Oerth at that point like we would see later).
The 3.0 Living Greyhawk Gazetteer was also posted this week: