Here is the full list of all 26 The Dark Side of the Moon achievements.
You got Dean killed for real.
You offended Blackwell.
You made Gedeon reveal himself.
You got ending number 2!
You attacked Julian!
You got ending number 3!
You called Alyx while with her.
You had a catchup with Julian.
You trusted Julian.
You found the homeless man dead.
You trusted Alyx with no real hesitation.
You gave the homeless man money.
You said that you don't believe in God.
You found an Easter Egg!
You said goodnight to the kids first.
You made it to the end...or did you?
You stopped Gedeon!
You got ending number 1!
You called Alyx at the right time!
You made it to the moon!
You discovered Sarah's secret dungeon!
You trusted Blackwell over Alyx.
You shouted at the homeless man!
You said that you believe in God.
You gained access to the kids' laptop.
You took out the rubbish first.