For those that already own some of the DLC for Batman™: Arkham Knight, I'm aware that this includes all DLCs available on the store. I leave it to your personal judgement as to whether you should enter for those you're missing.

After checking the archive, this is apparently one of those that you can enter even if you own it, so I will be sending the winner a friend invite on Steam in order to confirm lack of ownership.

As far as I know the key has no region restrictions.