Usually (but not necessarily) by force. But in any event, not by statutorily legal lines of succession.
Tyranny
"Usurped" means to take a position of power or importance illegally or by force. It often refers to someone seizing control or authority that rightfully belongs to someone else. In a broader sense, it can also apply to taking over any role, function, or resource in an unjust manner.
Usually (but not necessarily) by force. But in any event, not by statutorily legal lines of succession.
If you mean, what is such a person called, the word is "usurper". If you mean, what is a name of a person who did that, Napoleon would be an example. He grabbed power by overturning the existing Government of France in 1799.
Usurper
A tyrant
homos
Cyprus
After seizing power, the Bolsheviks became the Communist Party of the Soviet Union.
One leader (tyrant) with absolute power. He/she seizes power illegally or by force.
no tyrants seized power illegally
Iran