This is sort of what Im looking for as well, perhaps coupled with some sort of dungeon level, somehow hidden for the last 10000 years, but still able to point on to fact that the elves left and directly or indirectly point to the cult thats trying to gate them back in.
That's fairly easy.
So, in my game Elves defend their land by placing around it layers of magical wards - basically traps. Since the elves live for 1000's of years, are magically adept, and do this all the time (they are maintained by Wardens, the closest thing the elves have to police and a standing army), they can literally create millions of wards over time. Most of the traps are subtle, and designed to thwart but not kill trespassers. Most commonly these serve to confuse sense of direction, sense of time, and memory. A typical well developed system of wards will include barriers that block summoned creatures, wards that raise obscuring fogs around travelers, wards that alter sense of direction so that those effect think they are walking in straight lines when in fact they are walking in circles, wards that alter sense of time so that they feel they've been walking for hours when they've only been walking for minutes, wards that cause people to forget why they were here, wards that raise ghostly sounds, wards that provoke fear, or wards that suggest the trespasser leave the area for various reasons. A typical ward 'wall' will be a mile or more thick, and some of the larger ones go for hundreds of miles. Most wards are tied to trees, but in areas with more prepared defenses they are carved into standing stones. In some cases a little rune is carved. In others, it is visible only through detect magic. Really sophisticated defenses could include things like Symbols, Gylphs of Warding, and other sorts of potent traps.
Smart trespasser's in my game world typically realize what they've walked into, and quickly go the other way, as elves typically shoot first and ask questions later if they find you within their borders. In your case though, the wards could persist long after the culture that created them has disappeared, so that after a while no one completely remembers what created the wards in the first place. The interior might not have living elves, but it might well contain a large number of haunts, apparitions, phantasms, ghosts, ghasts, and banshee haunting the ruins of the former civilization. Likewise, your ward wall may have 'decayed' as trees or other objects the wards were tied to died, leaving only these scattered moss covered standing stones and the traps that they represent.
Have you seen the Hobbit? If you assume the elven palace is carved into some caves somewhere similar to Thranduil's halls in the Greenwood, but in a Rivendell like vale surrounded by Doriath's wall of ward and off in a wilderness somewhere, you could conceivably have a very plausibly hidden area that no one or almost no one in the last 10000 years has penetrated. It wouldn't take too much effort to hide it sufficiently well that the PC's couldn't find it either without the help of the cultists.
If it were me, I'd give some thought to whatever gods the elves used to worship and what they think about all this. A plot to restore the elven race in a damned form is a plot that I think gods no matter how otherwise prone to keeping hands off would have to intervene in to some extent, one way or the other. Either all the elven gods are dead (in which case, Dead Gods trope), or else I'd start weaving their agents into the plot Dragon Lance style.
