The emphasis in Master Xehanort's line makes it really hard to understand what he's trying to say. o_o
It sounds something like, "Behold... these lifeless Keys you see used to be full of power... united with the hearts of their masters... as a great Keyblade war raged!"
The way he says it makes "united with the hearts of their masters" and "as a great Keyblade war raged" sound like unconnected sentence fragments. Actually, the grammar just sounds off in general.
Is the intention there to say that the Keys used to be both full of power and united with the hearts of their masters (ie., the masters and their Keyblades fought as one) but they're not anymore, or is it supposed to mean that the Keys used to be full of power, but they united with the hearts of their masters (ie., the masters died, leaving behind their Keyblades) because of the Keyblade Wars? The words aren't quite right for either, so I don't quite get it. =/