Belial is the Sub-Boss of Takamagahara Shrine (A-2). To reach this room, you need the Cog of Antiquity.

This boss room has exploding gas, which can be dismissed if you use the Egg of Creation in the room. If you do that, remember to unequip Flame Torc because it makes your hits not hurt Belial. It's possible to stand at the highest platform on either side of the room and throw Rolling Shurikens with proper timing.

It is also possible to fight Belial without the Egg of Creation, by using the Lamp of Time.