well, I usually turn to Zelda Wiki, as I find it is often the most accurate and well stocked. they also have well reasoned fan theories, and those are often quite interesting to read.
in the case of the death sword, all you can do is theorize, as you cannot find any mention of him in the game. in my opinion, the theory they have written on his wiki page sounds very plausible: zeldawiki.org/Death_Sword
there may be other Zelda sites that are more official and streamlined than Zelda Wiki, but, like you, I haven't looked all that hard