Here is the full list of all 37 Mortal Shell achievements.
Complete Mortal Shell
Defeat The Unchained
Transform the Axatana between different modes
Defeat Crucix, the Twiceborn
Unlock all of the shells in the game
Defeat Tarsus, the First Martyr
Bring a gland to Dark Father
Defeat Imrod, the Unrepentant
Pick 50 mushrooms
Petted Vlas's cat
Find your first Shell
Beat the game without ever bonding with a shell
Defeat Hadern in the tutorial
Unlock all secret lore
Kill an enemy with bear trap
Beat the game in obsidian dark form
Waste your life away with Baghead
Become familiar with all items
Kill an enemy by kicking them off a platform
Jam with a brigand
Spend over 50, 000 Tar in Vlad's Shop
Parry over 100 attacks
Defeat all different versions of Grisha in the game
Kill Two or More Enemies with one shot from the Ballistazooka
Kill an enemy with a hardened plunge
Renounce humankind
Spend 100,000 tar in The Virtuous Cycle
Return your first seed to Twin Sister
Return all seeds to Twin Sister and break the Virtuous Cycle
Unlock all Shell Shades for The Virtuous Cycle
Max familiarity with all of the game's lutes
Acquire 100 instincts in one cycle
Return a seed with Hadern in The Virtuous Cycle
Acquire all 4 weapon Instincts for the Axatana in one cycle
Complete a run in The Virtuous Cycle with the Dark Form Obsidian Shell Shade
Beat all variations of the Dark Shells in The Virtuous Cycle
Fully Upgrade Hadern's skill tree