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