  We present a spectroscopic analysis of Type I superluminous supernova
(SLSN-I), SN 2018bsz. While it closely resembles SLSNe-I, the multi-component
H$\alpha$ line appearing at $\sim30$ d post-maximum is the most atypical. The
H$\alpha$ is characterised by two emission components, one at $+3000$ km/s and
a second at $-7500$ km/s, with a third, near-zero velocity component appearing
after a delay. The blue and central components can be described by Gaussian
profiles of intermediate width, but the red component is significantly broader
and Lorentzian. The blue component evolves towards lower velocity before fading
at $100$ d post-peak, concurrently with a light curve break. Multi-component
profiles are observed in other hydrogen lines including Pa$\beta$, and in lines
of Ca II and He I. Spectropolarimetry obtained before (10.2 d) and after (38.4
d) the appearance of the H lines show a large shift on the Stokes $Q$ -- $U$
plane consistent with SN 2018bsz undergoing radical changes in its geometry.
Assuming the SN is almost unpolarised at 10.2 d, the continuum polarisation at
38.4 d reaches $P \sim1.8\%$ implying a highly asymmetric configuration. We
propose that the observed evolution of SN 2018bsz can be explained by highly
aspherical CSM. After the SN explosion, the CSM is quickly overtaken by the
ejecta, but as the photosphere starts to recede, the different CSM regions
re-emerge producing the peculiar line profiles. Based on the first appearance
of H$\alpha$, we can constrain the distance of the CSM to be less than $430$
AU, or even lower ($<87$ AU) if the pre-peak plateau is related to an eruption
that created the CSM. The presence of CSM has been inferred for other SLSNe-I.
However, it is not clear whether the rare properties of SN 2018bsz can be
generalised for SLSNe-I or whether they are the result of an uncommon
evolutionary path, possibly involving a binary companion.
