Ok, I've fixed this the way Irina said. Just go to your Nvidia control panel and change the 3D configuration (Nvidia Optimus) to make Heroes VI use always the Nvidia card and not the integrated Intel HD family.
Moderador de HeroesV.es
Actually I found this thread in trying to figure out a way to fix my own problem which happened to be the same as the one posted above.
Then I noticed this instruction: "go to your Nvidia control panel" however....
I have an intel graphics card.
So I figured there must be a way to fix this without having NVIDIA.
So what I did was when my screen went all dark, I opened up the options and changed the graphics resolution and hit apply.
VOILA. The lighting returned to normal.
Then I set my resolution back to normal and continued.
Hope this helps all you Intel users.
What did you change resolution to?