Teeth.

Heh-heh, all that's left is to come up with a race of pickled organs:D:

Level 1: appendix;
upgrade - large intestine (2 times more HP and the ability "to crap": reduces the morale of the attacked unit by 2).
Level 2: kidneys - archers, they throw "stones";
upgrade - bladder (archer, ability "to endure": increases its own HP by 3 times, but there is a 10% chance of failing, in which case the entire stack is lost).
Level 3: stomach - ability "to absorb" with a 25% chance: the attacked unit loses the ability to move around the battlefield and fight with any other unit, except for the stomach that absorbed them, for 2 turns;
upgrade - ulcerated stomach (adds acid damage to absorbed units).
Level 4: heart - manages to attack twice;
upgrade - "valentine" (when attacking, it can cast hypnosis with a 5% chance - like, to make them fall in love).
Level 5: triceps;
upgrade - biceps (ability "demonstration": pumped-up biceps "show themselves", as a result of which enemy units involuntarily surround them from all sides without attacking).
Level 6: lungs - they fly;
upgrade - smoked lungs (ability "terrible cough": after using this ability, the ability to shoot appears for 2 turns).
Level 7: spinal cord - flies, ability "paralysis" (the unit that destroys the stack of spinal cords remains chained to one place until the end of the battle);
upgrade - brain (attacks all standing nearby without counterattack, ability "to mess with the head": each turn, the magic power of the enemy hero decreases by 2 units down to 1, after a successful battle it is restored, however, if they flee, surrender, or are defeated, it remains reduced):D.

P.S. I apologize if this is off-topic and too vulgar...