It's called Declaration of War, allegedly an unreleased OST. I found it on youtube by side while going through Code Geass tracks researching for another song, and I remembered it when I heard it in the video you linked to. It is somewhat similar to Naruto's Orochumaru theme.
I found it finally, it's called Birthplace and I found it after a long time of unsuccessful consumption of code geass music. I decided to, perhaps ironically, attempt to look it up by looking at the names at the Code Geass Wiki, in hopes what a theme of the title will match the required song. It's not on youtube, so I could not possibly have known of it's ...
Edit/update: I found the song what is identical to the one I was looking for. It's titled Out of the Ordinary. The one below is somewhat similar to this track, though.
I found one that appears to be similar to the one I was looking for, it's called Cold Nobility, I suppose it might even be supposed to be the same song, despite some differences.