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Superluminous supernovae are among the most energetic stellar explosions inthe Universe, but their energy sources remain an open question. Here we presentlong-term observations of one of the closest examples of the hydrogen-poorsubclass (SLSNe-I), SN~2017egm, revealing the most complicated known luminosityevolution of SLSNe-I. Three distinct post-peak bumps were recorded in its lightcurve collected at about $100$--350\,days after maximum brightness, challengingcurrent popular power models such as magnetar, fallback accretion, andinteraction between ejecta and a circumstellar shell. However, the complexlight curve can be well modelled by successive interactions with multiplecircumstellar shells with a total mass of about $6.8$--7.7\,M$_\odot$. In thisscenario, large energy deposition from interaction-induced reverse shocksresults in ionization of neutral oxygen in the supernova ejecta and hence amuch lower nebular-phase line ratio of [O\,\textsc{i}]$\lambda6300$/([Ca\,\textsc{ii}] + [O\,\textsc{ii}]) $\lambda7300$ ($\sim 0.2$)compared with that derived for other superluminous and normal stripped-envelopeSNe. The pre-existing multiple shells indicate that the progenitor ofSN~2017egm experienced pulsational mass ejections triggered by pair instabilitywithin 2 years before explosion, in robust agreement with theoreticalpredictions for a pre-pulsation helium-core mass of 48--51\,M$_{\odot}$.Finally, this work shows that the final explosion product may be a black holewith about 40\,M$_{\odot}$, and has significant implication for the formationof such heavy black holes that have been recently observed by LIGO-Virgogravitational wave detectors.

A Superluminous Supernova Lightened by Collisions with Pulsational Pair-instability Shells