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