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