I really don't see why Edge wouldn't still have it after s3 and seeing as how he kinda said/did next to nothing in s3 (which was one of the few let-downs I had with that game) it wouldn't be too hard to do something neat with him.
As to the fact that ol' Zodiac isn't in 5 and Zerase references a night sword (which for all I know could be a poor translation for star/zodiac) is pretty clearly a reference. Unless lesser versions can be made of the true night rune in much the same way that lesser versions could be made of the other runes (like fire, lightning etc.) but as far as I know we've seen no indications of this.
What it goes after at the end of 3? I always just assumed it was after some other source of undead baddies. I mean, that's kind of it's thing. Right?
But on the topic of the Star Dragon Sword, it just occurred to me that I can't remember if there was any part in s3 when Lilly remembers it or if it recognizes her at all, but it's been kind of a long time. Anyone remember if anything like that ever happened?
Still waiting for that daring piece of fan art with Zahak and Galleon sharing eyeliner in the bathroom.