The Deviljho patch went live last night. If you've completed the ??? Rathian special investigation in high rank, you will get a notification that a Devlijho has been spotted. Go to the expedition map and look for an area with a ??? monster. Launch into an expedition on that map and you will be able to find Deviljho footprints and find (and fight, if you wish) the Deviljho. After you return to Astera you'll be given a special investigation for the Deviljho.
It was a pretty fun fight. He is very large, very fast, and very aggressive. I couldn't even really hit him with a great sword, switched to my bow with better results. From experience with past games, if you have a poison weapon it will be very useful against him.
Every weapon type has a Deviljho weapon, and many of them are very good, and all of them look great. Armor could be better but has multiple pieces with handicraft which may be helpful for set building.