せう

The historical kana rendering せう was formerly read as /ɕeu/. Over time, this pronunciation shifted to /ɕoː/, and during the spelling reforms of the Showa era, the kana spelling was changed to しょう to match the pronunciation. There is nothing in modern Japanese that is read as せう, with a modern pronunciation of /seu/.

This reading せう is only used historically. It has been superseded in modern Japanese by the reading しょう (shō). See that entry for characters read as しょう (shō) in modern Japanese.