Now i just helped Septimus inscribe the lexicon, as i was leaving i had the wretched abyss portal appear and ask me whether i wanted to join him.

What happens if i select yes or no? I'm inclined to select no, but worried i may miss out on something?

Nothing really happens at the end of the quest with it, it speaks to you again then goes, haven't seen it again. But I chose yes at the first encounter, you might have to fight something if you say no, I have no idea.