Why is he here? According to executive producer Wendy Mericle, Prometheus will serve a purpose beyond just giving Star City trouble. Here's what she had to say about why he was chosen for the fifth season villain:

I think for Season 5, just because it is an answer to Season 1, and we are sort of closing this chapter out, we did want to go back to our roots and figure out what is specific to this show. What is our universe? Go back to 'What is Arrow?'

Wendy Mericle's tease in her chat with IGN proves that Arrow will be embracing its darker early days as Oliver's five-year journey comes to a close, and Prometheus' prime motivation for his villainy is a classic: revenge. The time has come for him to act on his anger toward Oliver and strike out as violently as necessary. He'll be connected to Oliver in a way that could make him even more of a formidable foe in Star City than either Ra's al Ghul or Damien Darhk, and Prometheus as the big bad could add an element of narrative symmetry to the first five seasons of the show.