Here is the full list of all 14 Opus Mortem achievements.
Collect 100,000 souls
Maximize Temperance
Maximize Resilience
Collect 10,000 souls
Collect 10 souls
Maximize Chrono's Will
Maximize Equilibrium
Maximize Evil Souls
Maximize Expertise
Maximize Greed
Maximize Patience
Maximize Pure Souls
Maximize Sloth
Reach combo x9