I sent this out to my players, prior to the upcoming session. This is their first in-game information about the megadungeon they can eventually go and explore:
You also hear a few rumors about the ruins to the north. There is a
"lone" mountain (actually a peak plus a few surrounding big hills) to
the NE of here, with a ruined fortress on top and legends of extensive
caves and dungeons underneath. Locals call it "the Ruins" (for obvious
reasons) and "Felltower" (for reasons that aren't clear), but it's
also known by other names. The orcs apparently call it Grak Yorl,
which locals say means either "the Black Fortress" or "the Boneyards"
depending on who you ask. The ruins are cursed (again, according to
rumor) and what's underneath is anyone's guess. It's officially
off-limits to go there, FWIW.
I've told them before, out of game, that there is a megadungeon to the north, and that it's essentially there to explore. I've also told them flat out it wasn't ready yet, and to concentrate (for now) on rooting out the evil shrine in one of the caves of the Caves of Chaos. Once that's done they can keep raiding the caves, investigate the odd and wicked swamp to the southeast, or head into the dungeon.
I'm basically giving them a limited sandbox. Basically saying something like "The adventures are in this area for now. Within these bounds, do whatever. I'll expand the bounds later." I'm just starting to give out the rumors about the megadungeon, to prep the grounds for play.
I'm excited, because although I've played RPGs (starting with red box Basic D&D) since 1981, I never actually ran a megadungeon. Or played in one.
(And thanks to Doug Cole for suggesting the name Felltower. I won't explain here why it fit so well.)