So this "future descendant" of Grohmul Djun caused problems for the Wavedancers during the 'Wavedancers' arc.
Except he was not really a future descendant of Grohmul Djun - as we know the 'Wavedancers' story didn't take place centuries after Shards but shortly after, or even parallel with, that storyline.
Anyway: Ardan Djarum was not a future descendant of Grohmul Djun, he was simply his son, a bastard. He was just crazy and thought he was from the future.
To prove it he stole a gun, and then his half-brother Angrif took it back, and captured Ardan, tortured him, and eventually shot him.
Ardan Djarum how he was pictured by Craig Taillefer as nemesis of the Wavedancers:
"Storms rage within this human sea captain, fueled by crimes - real or imagined - done to him and his by the WaveDancers. If he must destroy the sea itself to rid Abode of the mer-elves, he will not hesitate."
Just a thought: The bastard son who thought he was from the future thing also explains how 'Djun' became 'Djarum'.
Sure, 'Djun' is a title rather than a name; Grohmul was 'Djun', Angrif is 'Djun', Lodok and Rathol were never 'Djuns'. However, I think it's safe to assume that nobody has surnames in the modern (western World) sense of the Word, so Angrif is called Angrif Djun (bit like 'Rex' is sometimes used as a "surname" for a king - or "Regina" for a queen). Maybe Ardan (pre-crazy) knew that he couldn't actually go around calling himself 'Djun' so he went with the next-best choice and calling himself something that was similar-yet-different enough to 'Djun' that he felt it reflected his legitimacy, but the Djun wouldn't think he was trying to claim power.
^ And that's what happens when I attempt to write a complicated post at almost 11 PM...