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