The Mad Scientist

It begins when you talk to Logan in Guild Upper Level 3, once you've completed Destroy Vados.

It lists no rewards of its own, so finishing it pays nothing but its reputation.

It's one of four quests, with Playing with Salem, Deena Is Reborn and Finding Vados, that must all be complete, after a certain point in the story, before the game counts you as ready for the Abyssal realm.

How quests move on, pay out and count toward your reputation is on the Quests page.