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