tyrant

An extremely oppressive, unjust, or cruel ruler.

An absolute ruler who governs without restrictions, especially one who seized power illegally.

An oppressive, harsh, arbitrary person.

To tyrannize over.

To play the tyrant; tyrannize: sometimes with indefinite it.

In ancient Greece, an irresponsible chief or magistrate with unlimited powers, owing his office primarily to insurrection or usurpation.

Hence A wilfully arbitrary monarch or person in authority; a ruler or master who uses his power cruelly or oppressively; any person who treats those bound to him in any way as slaves to his will; an autocratic oppressor.

A tyrannical or compulsory influence; something that constrains the will inexorably; an overruling power.

In ornithology, a tyrant-flycatcher; one of the Tyrannidæ.

To act like a tyrant; to play the tyrant; be to tyrannical.