That's a Hagure Magin, this is a random boss that has bigger chance appearing when you have low control over the map (the gauge on the top right). Killing them yield good loot as far as I remember but are not essential, you usually can get it some other way. Most people would not find one in their gamethrough since the probability when you have nomal control is very low.
There are a few in the game.If you want to spawn one try staying on top of a heal feather and ring the bell hundreds of times. It can be pretty boring though.