RAGtime - the series of the Relativistic Astrophysics Group workshops and proceedings – started in 1999 at Institute of Physics, the Faculty of Philosophy and Science of the Silesian University in Opava, Czech Republic. The 21th RAGtime workshop took place at the Silesian University in Opava, Czech Republic on September 16-20, 2019. The scientific focus of the workshop was traditionally devoted mainly to problems of relativistic physics of black holes and neutron or quark stars. Large attention was given to confronting theoretical models with up-to-date observations available through both electromagnetic and gravitational wave window to the Universe; to the modelling of astrophysical effects in the combined strong gravity and relevant external magnetic fields; and to current issues in accretion theory. Problems related to cosmology, mathematical aspects of the theory of relativity, and alternative theories of gravity was addressed also. In addition, new methods of generating complex general relativistic solutions containing a black hole was exposed. Last but not least, special focus was putted on national and international collaboration regarding the present and future cosmic X-ray missions. The invited speakers included: Hackmann, Eva (ZARM, University of Bremen); Kluźniak, Włodek (Nicolaus Copernicas Astronimcal Center, Warsaw, Poland); Karas, Vladimír (Astronomical Institute, Academy of Sciences of the Czech Republic, Prague, Czech Republic); Rueda, Jorge A. (ICRANet, Pescara, Italy; ICRA, Dipartimento di Fisica, Universita di Roma Sapienza, Rome, Italy and INAF, Istituto de Astrofisica e Planetologia Spaziali, Rome, Italy); Ruffini, Remo (ICRANet, Pescara, Italy; ICRA, Dipartimento di Fisica, Universita di Roma Sapienza, Rome, Italy and INAF, Rome, Italy); Wielgus, Maciek (Center for Astrophysics at Harvard & Smithsonian, Cambridge, Massachusetts, USA and Black Hole Initiative at Harvard University, Cambridge, Massachusetts, USA)