Axiomatic Mind. The tertian can't be compelled to act in a manner contrary to its nature or its instructions. 

Disintegration. If the tertian dies, its body disintegrates into dust, leaving behind its weapons and anything else it was carrying.

Ultimate Inquisition. The tertian can innately cast detect thoughts at will, requiring no material components. The area around it is affected by a zone of truth spell. Its spellcasting ability is wisdom (DC 18). It can always tell when someone is lying to it.

Multiattack. The tertian makes two spear attacks and one tail attack.

Spear. Melee Weapon Attack: +11 to hit, reach 15 ft., range 50/100 ft., 1 target. Hit: 60 (10d12) piercing damage. 

Tail. Melee Weapon Attack: +11 to hit, reach 10 ft., 1 target. Hit: 80 (8d20) bludgeoning damage.

Gore. Melee Weapon Attack: +10 to hit, reach 5 ft., 1 target. Hit: 30 (5d12) piercing damage.

The tertian can take 4 legendary actions, choosing from the options below. Only one legendary action option can be used at a time and only at the end of another creature's turn. The tertian regains spent legendary actions at the start of its turn.

Gore. The tertian takes the gore action.

Question. The tertian casts detect thoughts.

Spear (Costs 2 Actions). The tertian takes the spear action.