Here is the full list of all 40 Pillars of Sorrow achievements.
Defeated 16 Enemies in W1
Defeated 66 Enemies in W1
Defeated 166 Enemies in W1
Defeated 16 Enemies in W2
Defeated 66 Enemies in W2
Defeated 166 Enemies in W2
Defeated 16 Enemies in W3
Defeated 66 Enemies in W3
Defeated 166 Enemies in W3
Defeated 111 Enemies across all Worlds
Defeated 333 Enemies across all Worlds
Defeated 666 Enemies across all Worlds
Got 6 Hearts out of Pets
Got 16 Hearts out of Pets
Got 66 Hearts out of Pets
Collected 6 Light Souls
Collected 6 Dark Souls
Collected 66 Light Souls
Collected 66 Dark Souls
Collected 166 Light Souls
Collected 166 Dark Souls