You have to look at the whole sentence to understand:
则是 contrasts the actions of 聂离 (he sat down next to the two Ye's), and 段剑 (he stood next to 聂离). English might use a conjunction such as "while", or "on the other hand".
则是 is not really a conjunction, though. It is more like what is sometimes called a "pseudo-cleft" sentence: "As for Duan Jian, what he did was to stand next to Nie Qin with a stern expression."
I don't think 而 is absolutely necessary here. If you don't use it though, I would probably put a period, rather than a comma.
This is a good example of how English and Chinese have different ways of linking compound sentences to form a narrative structure.