  Cosmic History has witnessed the lives and deaths of multiple generations of
massive stars, all of them invigorating their host galaxies with ionizing
photons, kinetic energy, fresh material and stellar-mass black holes.
Ubiquitous engines as they are, Astrophysics needs a good understanding of
their formation, evolution, properties and yields throughout the history of the
Universe, and with decreasing metal content mimicking the environment at the
earliest epochs. Ultimately, a physical model that could be extrapolated to
zero metallicity would enable tackling long-standing questions such as "What
did the First, very massive stars of the Universe look like?" or "What was
their role in the re-ionization of the Universe?".
  Yet, most our knowledge of metal-poor massive stars is drawn from one single
point in metallicity. Massive stars in the Small Magellanic Cloud (SMC, $\sim
1/5 Z_{\odot}$) currently serve as templates for low-metallicity objects in the
early Universe, even though significant differences with respect to massive
stars with poorer metal content have been reported.
  This White Paper summarizes the current knowledge on extremely (sub-SMC)
metal poor massive stars, highlighting the most outstanding open questions and
the need to supersede the SMC as standard. A new paradigm can be built from
nearby extremely metal-poor galaxies that make a new metallicity ladder, but
massive stars in these galaxies are out of reach to current observational
facilities. Such task would require an L-size mission, consisting of a
10m-class space telescope operating in the optical and the ultraviolet ranges.
Alternatively, we propose that ESA unites efforts with NASA to make the LUVOIR
mission concept a reality, thus continuing the successful partnership that made
Hubble Space Telescope one of the greatest observatories of all time.
