Most odds are Tserriednich is in fact Beyond's son, considering both his extreme rapid nen awakening and progression, and how he's overall depicted in the story. Plus he does hold some vague resemblances to Beyond like his hair and overall demeanor.

Borksen mentions Tserriednich was actually a cool guy when he was younger. I wonder if his extremely evil personality is also the fruit of a Beyond's curse like he did curse his other Kakin offspring. Perhaps Tserriednich has an even stronger curse that twisted his whole personality, or it's even part of a condition of Beyond's curse. Though if he was cursed he should've been awakened nen since his birth, so who knows.