This is Grammeni Oxia, apparently Europe's southernmost beech trees forest.

Grammeni Oxia means "pretty beech tree", although we don't really use the word "grammeni" anymore to describe something which is pretty.
It's a large, thick forest up on the heights of Mt Sarandena, neighbouring the mountains of Evrytania, Oeta, Vardousia, Giona and plenty of other mountains in Central Greece. There is a village in the area called Grammeni Oxia itself. This place is a 4-hour drive from Athens and in my opinion, one of the most complete mountain road trips you can have, since there are tons of places you can combine with it, such as the villages of Oeta and Karpenissi.

The first time I visited I stayed in Mikro Horio, Karpenisi. It's a 1-hour-something drive from there. The second time I visited I stayed at the mountain refuge of Grammeni Oxia, called Lambros Eftaxias. It reopened last year after a 10-year closure.
There are lots of hiking trails starting from that point, including the trail to get to the summit of Mt Sarandena, a relatively easy hike. Though, if you want to enjoy the forest and the amazing fall colours, your best bet is to drive from the village of Gardiki to the village of Grammeni Oxia.

Note that the last couple of kilometres before reaching the village of Grammeni Oxia is a dirt road and potentially unsuitable for regular cars, especially after a heavy rain or in icy conditions. As for the drive through the forest, I find no words to describe how pretty it is. It's shaded almost in its entirety due to the high trees and the silence is deafening. There are no houses or villages there. You might encounter hunters here and there, during hunting season.