  SN 2021adxl is a slowly evolving, luminous, Type IIn supernova with
asymmetric emission line profiles, similar to the well-studied SN 2010jl. We
present extensive optical, near-ultraviolet, and near-infrared photometry and
spectroscopy covering ~1.5 years post discovery. SN 2021adxl occurred in an
unusual environment, atop a vigorously star-forming region that is offset from
its host galaxy core. The appearance of Ly{\alpha}, O II, as well as the
compact core, would classify the host of SN 2021adxl as a Blueberry galaxy,
analogous to the higher redshift Green Pea galaxies. Using several abundance
indicators, we find a metallicity of the explosion environment of only 10%
solar, the lowest reported metallicity for a Type IIn SN environment. SN
2021adxl reaches a peak magnitude of r ~ -20.2 mag and since discovery, SN
2021adxl has faded by only ~4 magnitudes in the r band with a cumulative
radiated energy of ~1.5e50 erg over 18 months. SN 2021adxl shows strong signs
of interaction with a complex circumstellar medium, seen by the detection of
X-rays, revealed by the detection of coronal emission lines, and through
multi-component hydrogen and helium profiles. In order to further understand
this interaction, we model the H{\alpha} profile using a Monte-Carlo electron
scattering code. The blueshifted high-velocity component is consistent with
emission from a radially thin, spherical shell resulting in the broad emission
components due to electron scattering. Using the velocity evolution of this
emitting shell, we find that the SN ejecta collide with circumstellar material
of at least 5 Msun, assuming a steady-state mass-loss rate of 4-6e-3 Msun per
year for the first ~200 days of evolution. Continuing the observations of SN
2021adxl may reveal signatures of dust formation or an infrared excess, similar
to that seen for SN 2010jl.
