Here is the full list of all 39 Odium to the Core achievements.
Die 10000 times
Die 5000 times
Die 2000 times
Complete all nightmare levels
Achieve 100% completion
Play for 12 hours
Reach the target scores on all level
Unlock all skins
Defeat Gaudium
Find all the Cryptos in the game
Collect all the Orbs in the game
Die 1000 times
Play for 9 hours
Complete a level after dying over 100 times on it
Unlock the final level
Play for 6 hours
Unlock the first 5-skull level
Get hit by a train
Finish a 5-skull level on the first try.
Finish a 5-skull level dying once
Finish a 5-skull level dying twice
Die 500 times
Defeat Tristitia
Play for 3 hours
Watch the credits
Die 200 times
Defeat Timor
Collect all three gears in one run
Play for 1 hour
Die 100 times
Die 50 times
Die at 95% of a level.
Unlock a nightmare level
Reach the target score on a level
Die 10 times
Stay fully charged for 7 seconds
Collect all the Orbs in a level
Find all the Cryptos in a level
Finish the Tutorial