Here is the full list of all 20 Project Warlock achievements.
Complete the game on the hardcore difficulty
Find the one who was supposed to rip and tear until it was done
Complete the game on the easiest difficulty
Complete the game on the standard difficulty
Slay the Satan himself
Find your double-barrel-and-axe-carrying friend
Buy 10 upgrades
Destroy the Massive Tank
Find that gum-chewing and ass-kicking guy
Find 50 secrets
Slay the Sphinx
Find 30 Secrets
Find the guy who took everything too seriously
Ph'nglui mglw'nafh Cthulhu R'lyeh wgah'nagl fhtagn
Find that weird Chinese guy
Slay The Sorcerer
Find 10 Secrets
Find that guy who lived again
Find your first secret