Rabbi Mordchai Rosen was head dean, and grand rabbi of the village of Monistrich, known as Monastyrysche in Ukrainian, a town presently in Cherkasy Oblast (province) of Ukraine. He perished during the Holocaust in World War II and was survived by his sons R` Nachman and R` Froim.
The biography of this rabbi can be found in many Hebrew religious books, among them "Ner Nachman".
References
Monistritch (Hasidic dynasty) Wikipedia(Text) CC BY-SA