How can I reach the massive tree in the south, near Bald Mountain

So I am working my way around the south of the map clearing up the question marks in Velen. I've noticed the question mark which I pictured below, but having done a near full circle of the mountain, I cannot find a path up this mountain.

With the use of spoiler tags where necessary, how can I visit this location? I'm currently lv11, and am wondering if it's a story location I'll visit later.

This is a location you can only visit during the main quest. It's a pretty late quest, closer to the end of the game.

As far as I know you can't get there before the quest. I did read about a few people managing to get there, but running into glitches as this wasn't intended. I would just ignore that location, it'll come up at some point during the main quest.