This is a 5th edition adventure for level 3 characters by Goodman Games.
From the publisher:
A forgotten cavern beneath a lonely hill hides secrets as old as the world. Strange phenomena haunt those who delve its depths. Both the wise and the mad venture within to call upon the cave’s ancient power—and not all reemerge. Now, the legend of the Cave of the Unknown reaches the ears of stalwart adventurers. Can they uncover the Cave’s secret history and succeed where others have failed? Or will their bones soon litter the darkness under that distant hill?
The Cave of the Unknown is an old-fashioned dungeon crawl designed for modern fifth-edition game mechanics. It is intended for a party of four to six 3rd-level PCs. The adventure can be easily inserted into any campaign or used as part of the Into the Borderlands reprint.