Why does the Sparda sword look so much like Soul Edge?

[King Avallach]

Come on, you've noticed it too.
The Sparda sword is almost a complete replica of soul edge, same shape and colour scheme, demonic power to boot.

Come on, the similarities are stark!

 

[Tony_Redgrave]

Well I think the reason is this:

Both Swords are wielded by guys who can destroy armies with one swing - therefore the swords must be large. Second, you have noticed that they are designed by the Japanese swords - meaning that they have a blade only at one of its sides. And the rest is just about it being demonic, some flesh/bones/eyes etc to make it more demonic and fearsome.