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We present detailed optical photometry and spectroscopy of the Type IInsupernova (SN) 2021qqp. Its unusual light curve is marked by a long gradualbrightening (i.e., precursor) for about 300 days, a rapid increase inbrightness for about 60 days, and then a sharp increase of about 1.6 mag inonly a few days to a first peak of $M_r\approx -19.5$ mag. The light curve thenturns over and declines rapidly, until it re-brightens to a second distinct andsharp peak with $M_r\approx -17.3$ mag centered at about 335 days after thefirst peak. The spectra are dominated by Balmer-series lines with a complexmorphology that includes a narrow component with a width of $\approx 1300$ kms$^{-1}$ (first peak) and $\approx 2500$ km s$^{-1}$ (second peak) that weassociate with the circumstellar medium (CSM), and a P Cygni component with anabsorption velocity of $\approx 8500$ km s$^{-1}$ (first peak) and $\approx5600$ km s$^{-1}$ (second peak) that we associate with the SN-CSM interactionshell. Using the bolometric light curve and velocity evolution, we construct ananalytical model to extract the CSM profile and SN properties. We find twosignificant mass-loss episodes with peak mass loss rates of $\approx 10$M$_\odot$ yr$^{-1}$ and $\approx 5$ M$_\odot$ yr$^{-1}$ about 0.8 and 2 yearsbefore explosion, and a total CSM mass of $\approx 2-4\,M_\odot$. We show thatthe most recent mass-loss episode can explain the precursor for the yearpreceding the explosion. The SN ejecta mass is constrained to be $M_{\rmSN}\approx 5-30\,M_\odot$ for an explosion energy of $E_{\rm SN}\approx(3-10)\times10^{51}\,{\rm erg}$. We discuss eruptive massive stars (luminousblue variable, pulsational pair instability) and an extreme stellar merger witha compact object as possible progenitor channels for generating the energeticexplosion in the complex CSM environment.

Multiple Peaks and a Long Precursor in the Type IIn Supernova 2021qqp: An Energetic Explosion in a Complex Circumsteller Environment