HUGE SPOILERS FROM SHADOW OF THE COLOSSUS. DO NOT READ IF YOU HAVEN'T PLAYED THE GAME
YOU HAVE BEEN WARNED
With every colossus you beat you become more and more pale (like a monster). About the colossus 12, there's a cutscene where you see some knights riding. Before the 16th, you cross with Agro a broken bridge, it crumbles, the horse kicks you and saves you, then falls trough a cliff (that scene, the "AGRO!!!!!!!!!!!" is fantastic)
When you beat the 16th, you go back to the shrine, at that moment, the knights enter and say "What've you done? You stole the sword and revived them (the dual god Dormin)" and they throw arrows at you that hit you everywhere. Still wounded, you try to walk to Mono (still asleep), but you fall. Then, a shadow emerges from you (kinda like the ones that appear around you when you beat a colossus), and Dormin attacks. The knights escape and decide to throw the sword in the pond to seal the gods. When they do, you (as a shadow) start being sucked into the pond. You can't escape it and disappear. Then Dormin revives Mono, before being sealed, Agro walks in with a broken leg, the girl walks to the pond and sees a baby with horns (where you should have been) and takes it. Seeing that she can't escape, supposedly they live there (and the boy with horns is the lineage that descends to ICO's protagonist)
More or less, that's the story