Ogien was the god of fire until Makuta was frustrated with him and sent in an assassin to kill him. He knew that if he left the body in plain sight, the Toa and Matoran could easily find it and reunite it with the Great Mask, so he hid it in the volcano of Ta-Metru. A few years later, the Fire Tribe settled there as you all know. They eventually found the body and reunited it with the Great Mask. And Ogien, God of Fire was born again, as you see him now.
That is a lot of Hero Factory armor parts! I like some of the different layers that you made with those Hero Factory armor parts. The only problem I have with the MOC is the design of the headset gear, it looks so weird. I mean, maybe it was the only solution as to incorporate the headset. The headset itself looks like it is sticking too far apart from the back of the head (you can definitely see what I am talking about in the first picture). Maybe you can use the Metru head (a different mold of head skeleton) and make a new headset. I'm sure there is a way to make the connection of Metru head and Hero Factory torso together, I just saw a solution on Google lol. Other than that, I'm digging the look of the creation. Nice job!