Up in the mountains in the ruins of a long lost civilisation is the Horn. A massive horn the purpose of which nobody knows. Only it has recently started to glow and night, and young apprentice wizards sent to look didn't come back...
Adventurers who are bold (or foolhardy) enough to go take a look will find themselves exploring a subterranean cave system, for which the GM is provided with a nice full-page map and short notes for what's found where.
If you and your players enjoy exploring caves and fighting the creatures they find there, this hangs together quite nicely. Those who want more of a plot will need to add it for themselves.
Can the adventurers find out why the Horn is glowing or what's happened to the young magelings? Up to you.