Unbreachable valley

Between the barren wasteland inhabited by the most cruel, and most stubborn tribes of the universe and the lands of the Empire, stood a tall divine mountain.
The mountain was said to be the spine of a divinity from the Empire, that fell against the hordes of myriad races trying to invade, essentially making the barrier that protects the Empire.
Nobody from the wastalands can survive the trekk across the mountain, for it is poisonous, and actively hostile towards them.
But there is a small valley, that is said to be where the divinity was fatally injured, that is a free land, where the cross between the two places can be done.
This valley is the only point of entrance for the the wasteland natives, other than outer space, where the Empire has overwhelming superiority.
But this valley is legendary, for it is known to be unbrachable, because since the creation of the mountain range, those in the wasteland never managed to step into the lands of the Empire.
Why?
Was it because of a great military defence?
No, because the baracks, fortresses and the military lines of the Empire, are way behind the mountain, not even close to the valley.
Then how is it possible, that the tribes that would rather die than to give up the prosperous lands of the Empire havent tried to cross the valley?
They have tried, but they never succeeded, for it is protected.
It is said that a sun is living the valley, protecting the Empire.
Nobody knows if this is a metaphor for someone of the royal family, or someone related to the divinity, but one thing is certain.
No enemy has ever managed to cross into the lands of the Empire, through that valley.
There are countless records of tries to do so, but all ended up in the same way: in failure.
For a bright flame is lit whenever someone not of the Empire steps into the valley.
And no matter how many water spells are thrown at the flames.
No matter how many people throw themselves into the flames.
No matter if entire landscapes are thrown into the valley.
The flames once burning, they don’t go out, until the valley is free of enemies.
The latest records state that an Emperor of the tribes, alongside with more than a trillion soldiers have “attacked” the valley.
The flames that appeared due to this incursion were so hot that even at million kilometres away, dry pieces of wood would catch on fire randomly.
That even at the darkest moment of the night, it looked like day.
It lasted for over a year, before a shout was heard, and the flames went out.
Later on, the Empire learnt that the Emperor survived, yet it was badly burnt, and it would take centuries to heal.
Ever since then, the Empire faces much more attacks in outer space, their fleet slowly getting matched by the enemy fledgling forces.
But no more attempts were made towards the valley.
And the people of the Empire rest easy, for the valley, is true to its name.