The God-Emperor

An imperial family developed a society that worshipped them as if they were gods. Over the span of several generations it became their core belief, with the Imperial family crushing and suppressing any other forms of faith/religion. As the Empire expanded and more of the populace worshipped the Emperor, his power grew until he became a God himself. Power corrupted him and he became tyrannical, oppressing his people. Underground religions formed, as they realised the effect of their worship and refused to add power to his rule. Those that were discovered were wiped out, entirely. As the Emperor acquired more power he was able to use it to cause fear and intimidation in his subjects and enemies.