Enderal. Marius' picture was from there too.
It "is a total conversion for TES V: Skyrim: a game modification that is set in its own world with its own landscape, lore and story. It offers an immersive open world, all for the player to explore, overhauled skill systems and gameplay mechanics and a dark, psychological storyline with believable characters."
Quite deep, big and beautiful, I like it better than Skyrim somehow, especially the story.
The team made similar mod for Oblivion: Nehrim.
Thanks to Llama for "finding" it.