Theseus is a mission proposal in response to the ESA call for medium size mission (M5). It is selected, on 2018 May 7, to enter the assessment phase study.
Theseus aims at exploring the early Universe by discovering and investigating Gamma-Ray Bursts in the first billion years and by performing a deep monitoring of the transient Universe in the soft X-ray domain. These goals will be achieved through a set of instruments allowing detections over a broad field of view, an energy band extending from several MeV down to 0.3 keV and high sensitivity to transient sources.
I give a description of the scientific goals, its instruments and develop some comparison with other ongoing or upcoming similar missions.