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We report the discovery of Mothra, an extremely magnified monster star,
likely a binary system of two supergiant stars, in one of the strongly lensed
galaxies behind the galaxy cluster MACS0416. The star is in a galaxy with
spectroscopic redshift $z=2.091$ in a portion of the galaxy that is parsecs
away from the cluster caustic. The binary star is observed only on the side of
the critical curve with negative parity but has been detectable for at least
eight years, implying the presence of a small lensing perturber.

Microlenses alone cannot explain the earlier observations of this object made
with the Hubble Space Telescope. A larger perturber with a mass of at least
$10^4$\,\Msun\ offers a more satisfactory explanation. Based on the lack of
perturbation on other nearby sources in the same arc, the maximum mass of the
perturber is $M< 2.5\times10^6$\,\Msun, making it the smallest substructure
constrained by lensing above redshift 0.3. The existence of this millilens is
fully consistent with the expectations from the standard cold dark matter
model. On the other hand, the existence of such small substructure in a cluster
environment has implications for other dark matter models. In particular, warm
dark matter models with particle masses below 8.7\,keV are excluded by our
observations. Similarly, axion dark matter models are consistent with the
observations only if the axion mass is in the range $0.5\times10^{-22}\, {\rm
eV} < m_a < 5\times10^{-22}\, {\rm eV}$.