So at the end of this you find a strange green glowing entity that the boy dubs the master of this place he seems to have control over the shadow people and is pulling the strings from his tower and his control stretches over the whole area. so pretty much the same power as the witch in ico has, same doors same shadow people same green magic stuff. both games have a story tied into sacrificing small boys and turning them into magic trico food and shadow people.
this got me thinking on how its linked to shadow of the colossus well, in that your tasked with slaying 12 colossus to return this being in the temples power again, we see shadow people in the temple and the same green power that is released in the form of tendrils on slaying the colossus, and the colossus have the same green glowing parts like Tricos horns, the difference here is that the Titans are used as a form of imprisonment holding this entity's power and keeping it weak, he has no control over them like he did the tricos and they remain dormant until challenged.
my theroy is that in all the games your fighting the same enemy we already know its an abstract being in shadow of the colossus and in the last guardian, and it shows how it can posses people and use there power as it dose to wander at the end of colossus so again that links nicely into the witch in Ico.
timeline wise simply put
this magical entity is first encountered at the end of the last guardian, and released, (it appears to be in very elaborate cage)
in shadow you encounter him as a being trapped and stripped of his power and you help him gain his power back
in Ico the being has gained power again and is seeking immortality by transferring itself to a perfect host