Below is a diagram which lists all of the Blackskull race evolutions. You can access these by siding with Blackskull and becoming a Sea Wretch.
To evolve you should seek out an Evolved Spirit NPC, there is one at 32, 62 (Knotwood Valley) inside the South Glade Tower, but there are others.
The rewards listed above are a one time thing applied when you evolve, there are no per level stat bonuses from evolving.