Someone who exerts their authority in a cruel or oppressive way is tyrannous. An unjust, autocratic leader of a country is a tyrannous ruler.
You might accuse your parents of being tyrannous if they don't let you go out on a school night, but unless they truly abuse their control over you, they're not literally tyrannous. A tyrant is an absolute ruler, often an illegitimate (rather than democratically elected) one, and is usually cruel. Both tyrant and tyrannous come from the Greek root tyrannos, "absolute ruler unlimited by law."