It's the Akatosh style, it's currently only available as the Flanking set which you get for completing daily bounties in Kvatch and Anvil.

damn i just looked at those sets and they are garbage. typical eso make something cool looking worthless, but they made alliance styles craftable so hopefully we can get this one. i mean there is 2 thieves guild styles i mean seriously cause the first one was so cool lets make 2 of them.......great it idea. should have made this one craftable and dropped the second thieves armor from release, a little more originial