  At metallicities lower than that of the Small Magellanic Cloud, it remains
essentially unexplored how fossil magnetic fields, forming large-scale
magnetospheres, could affect the evolution of massive stars, thereby impacting
the fundamental building blocks of the early Universe. We extend our stellar
evolution model grid with representative calculations of main-sequence,
single-star models with initial masses of 20 and 60 M$_\odot$, including
appropriate changes for low-metallicity environments ($Z = 10^{-3}-10^{-6}$).
We scrutinise the magnetic, rotational, and chemical properties of the models.
When lowering the metallicity, the rotational velocities can become higher and
the tendency towards quasi-chemically homogeneous evolution increases. While
magnetic fields aim to prevent the development of this evolutionary channel,
the weakening stellar winds lead to less efficient magnetic braking in our
models. Since the stellar radius is almost constant during a blueward evolution
caused by efficient chemical mixing, the surface magnetic field strength
remains unchanged in some models. We find core masses at the terminal-age main
sequence between 22 and 52 M$_\odot$ for initially 60 M$_\odot$ models. This
large difference is due to the vastly different chemical and rotational
evolution. We conclude that in order to explain chemical species and, in
particular, high nitrogen abundances in the early Universe, the adopted stellar
models need to be under scrutiny. The assumptions regarding wind physics,
chemical mixing, and magnetic fields will strongly impact the model
predictions.
