How did Wu Zhao come to power? Why was she so incredibly unique in Chinese Imperial history?

It is hard to tell you how she came into power, because I think the history is complex, so I suggest you to refer to https://www.travelchinaguide.com/intro/history/tang/emperor_wuzetian.htm for more information.
She was unique, because she was the first woman empress in Chinese history and under her ruling, Chinese society is prosperous.