Thank you very much for your help, Milo. Even though I don´t get what those symbols mean, I got the main idea of this swords. The problem was that both blades seem identical except in different colors, and I prefer having all different cards if possible. In Mugendai-Yaiba, I used that "the HP of the wielder defines its power" thing, wich is their original function. As it is Eternion´s original blade, I made it so that only black decks could use it.
For Kyuukyou-Yaiba, I had to come up with something different. This is what I did.
And in one of those comics you pointed me out, I found something that reassures me in the fact that termis should be the five color Eternal. In
here Ardam says that Termis has abilities from every other eternal.