Context : Low-metallicity environments such as the early Universe and compact star-forming dwarf galaxies contain many massive stars .
These stars influence their surroundings through intense UV radiation , strong winds and explosive deaths .
A good understanding of low-metallicity environments requires a detailed theoretical comprehension of the evolution of their massive stars .

Aims : We aim to investigate the role of metallicity and rotation in shaping the evolutionary paths of massive stars and to provide theoretical predictions that can be tested by observations of metal-poor environments .

Methods : Massive rotating single stars with an initial metal composition appropriate for the dwarf galaxy I Zw 18 ( [ Fe/H ] = - 1.7 ) are modelled during hydrogen burning for initial masses of 9-300 M _ { \odot } and rotational velocities of 0-900 km s ^ { -1 } .
Internal mixing processes in these models were calibrated based on an observed sample of OB-type stars in the Magellanic Clouds .

Results : Even moderately fast rotators , which may be abundant at this metallicity , are found to undergo efficient mixing induced by rotation resulting in quasi chemically-homogeneous evolution .
These homogeneously-evolving models reach effective temperatures of up to 90 kK during core hydrogen burning .
This , together with their moderate mass-loss rates , make them Transparent Wind Ultraviolet INtense stars ( TWUIN star ) , and their expected numbers might explain the observed He II ionizing photon flux in I Zw 18 and other low-metallicity He II galaxies .
Our slowly rotating stars above \sim 80 M _ { \odot } evolve into late B- to M-type supergiants during core hydrogen burning , with visual magnitudes up to 19 ^ { \mathrm { m } } at the distance of I Zw 18 .
Both types of stars , TWUIN stars and luminous late-type supergiants , are only predicted at low metallicity .

Conclusions : Massive star evolution at low metallicity is shown to differ qualitatively from that in metal-rich environments .
Our grid can be used to interpret observations of local star-forming dwarf galaxies and high-redshift galaxies , as well as the metal-poor components of our Milky Way and its globular clusters .