Who is Nana Sahib from Flashman in the Great Game: From the Flashman Papers 1856-1858 and what is their importance?

In 1857 to 1858, Nana Sahib acts as one of the leaders of the Sepoy Mutiny. Flashman negotiates with Sahib directly in order to secure the release of the barges, but when all of the barges are finally released, Sahib orders his men to open fire. One one barge escapes. Survivors are imprisoned and butched per Sahib's orders.