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