Here is the full list of all 20 TENET achievements.
Kill 100 Enemies
Score 10,000 PTS
Kill 1 Enemy
Decapitate an Enemy
Kill 1,000 Enemies
Kill 10,000 Enemies
Kill 100,000 Enemies
Kill 1,000,000 Enemies
Create 100 Gibs
Create 1,000 Gibs
Create 10,000 Gibs
Create 100,000 Gibs
Create 1,000,000 Gibs
Score 50,000 PTS
Score 100,000 PTS
Score 250,000 PTS
Score 500,000 PTS
Score 1,000,000 PTS
Unlock the Mooch Follower
Customize your Player