  The generation of frequency-tunable, narrow-bandwidth and
carrier-envelope-phase stable THz pulses with fields in the MV/cm regime that
can be appropriately timed to the femtosecond X-ray pulses from
free-electron-lasers is of highest scientific interest. It will enable to
follow the electronic and structural dynamics stimulated by (non)linear
selective excitations of matter on few femtosecond time and {\AA}ngstrom length
scales. In this article, a scheme based on superradiant undulator radiation
generated just after the XFEL is proposed. The concept utilizes cutting edge
superconducting undulator technology and provides THz pulses in a frequency
range between 3 and 30 THz with exceptional THz pulse energies. Relevant
aspects for realization and operation are discussed point by point on the
example of the European XFEL.
