Sviatoslav Tagon is a pseudonym of Symon Petlura, used from late 1904 to early 1905.
At the time, Petlura was living in Lviv, and when a general amnesty was announced, he moved to Kiev where he continued his independence work.
During this period of his life, Petlura worked with Ivan Franko and Volodymyr Hnatiuk in publishing Ukrainian literature.
He worked with the Shevchenko Scientific Society, the Literaturno-Naukovy Zbirnyk (Literary-Scientific Collection), as well as "Volya" magazine.
References
Sviatoslav Tagon Wikipedia(Text) CC BY-SA