Ultraluminous X-ray sources (ULXs) are some of the most energetic stellar objects in the Universe. The nature of ULXs is still heavily debated; they have been interpreted as stellar-mass black holes/neutron stars accreting at super-Eddington rates, massive stellar-mass black holes accreting around Eddington and even intermediate-mass black holes accreting at sub-Eddington rates. In order to differentiate between these potential models, a constraint on the mass of the compact object is required. Understanding the accretion/ejection coupling and spectral properties of these extremely luminous sources are also important open questions in ULX research.
Our recent discovery of the first eclipsing ULX will allow us to make progress on these unsolved problems. The presence of eclipses allows us to place constraints on the physical parameters of the binary system, such as the mass function and orbital period. Eclipses tell us that the source is seen almost edge-on; therefore, we can test the relationship between geometry and spectral appearance of ULXs. The source also shows the presence of a radio jet and coincident optical nebula. Recent spectroscopic data have allowed us to determine the dynamics of the gas and the mechanism behind the nebular inflation. All of this enables us to explore the inflow/outflow coupling and power distribution in the most luminous compact stellar remnants.