What secret Chinese group that rebelled against foreigners?

Answer 1

That almost sounds like the "Militia United in Righteousness" (Yihetuan), known in English as the "Boxers" - except that group was not secret. The group did absorb some Martial Arts societies that had been operating in secret but the Yihetuan operated quite openly. Modern reincarnations of the Yihetuan do operate as secret societies.

Turmoil from a severe drought, followed by flooding and some political unrest disrupted life for many of the poor in China. At the same time multiple foreign powers including England, France, Germany, Russia, and Japan were vying for influence in or outright control of in China . Foreign powers had defeated China in several wars, asserted a right to promote Christianity and imposed unequal treaties under which foreigners and foreign companies in China were accorded special privileges, extraterritorial rights and immunities from Chinese law, causing resentment and xenophobic reactions among the Chinese. At the same time, Christian missionaries were disrupting the established order by building Christian churches all over China and proselyting. In some cases they actually acquired former Temples and Shrines and re-purposed them as Christian Churches.

Into this chaos came the Yihetuan who were ultra-nationalist and sought to restore China to it's former glory by, among other things, purging it of foreign power and influence.