(Sorry if grammar is bad, using phone). Idea for thought, ven encounters tragedy of some sort, shock erases his memories at least for the time. Some how gets sent into dark realm and survives only by his own powerful light in his heart, which was could have been accompanied by darkness due to his long exposure in there. He only aged a little due to the dark realm having almost no time change. Eventually Xehanort, who had his own master train him in darkness, found ven and noticed his ratio of light to dark. Where he got the idea for the x-blade, beyond me. Also side note, yen sid possibly having book of phofecy due to being apprentice to the first person to receive book besides 5 apprentices? Food for thought