First of all, when you insert the sword, the pedestal opens up and reveals some sort of technology (same as when you beat the god king).
After that, you have two doors that open up. The first one contains this guy:
He is
very hard to beat. Mainly because one hit from him makes you lose most of your HP. I eventually did beat him, after which he blew up...
After that, I went through the next door. There, some guy who looked like some sort of samurai was just sitting there. He talks, like the god king.
It took me one try to beat him... he blows up too.
Next, a new door opens. It leads outside to some pit with some lizard-thing reading.
I didn't actually manage to beat it yet... but I will eventually.
By the way, every time I beat one of these bosses, they gave very rare items.
If I get any further, I'll post some more pics.