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We present photometry and spectroscopy of the slowly evolving superluminousType IIn SN2015da. SN2015da is extraordinary for its very high peak luminosity,and also for sustaining a high luminosity for several years. Even at 8\,yrafter explosion, SN2015da remains as luminous as the peak of a normal SNII-P.The total radiated energy integrated over this time period (with no bolometriccorrection) is at least 1.6 FOE. Including a mild bolometric correction, addingkinetic energy of the expanding cold dense shell of swept-up circumstellarmaterial (CSM), and accounting for asymmetry, the total explosion kineticenergy was likely 5-10 FOE. Powering the light curve with CSM interactionrequires an energetic explosion and 20 Msun of H-rich CSM, which in turnimplies a massive progenitor system above 30 Msun. Narrow P Cyg features showsteady CSM expansion at 90 km/s, requiring a high average mass-loss rate ofroughly 0.1 Msun/yr sustained for 2 centuries before explosion (althoughramping up toward explosion time). No current theoretical model for single-starpre-SN mass loss can account for this. The slow CSM, combined with broad wingsof H$\alpha$ indicating H-rich material in the unshocked ejecta, disfavor apulsational pair instability model for the pre-SN mass loss. Instead, violentpre-SN binary interaction is a likely cuprit. Finally, SN2015da exhibits thecharacteristic asymmetric blueshift in its emission lines from shortly afterpeak until the present epoch, adding another well-studied superluminous SNeIInwith unambiguous evidence of post-shock dust formation.

SN~2015da: Late-time observations of a persistent superluminous Type~IIn supernova with post-shock dust formation