Observational properties of extreme supernovae

The past ten years have opened up a new parameter space in time-domain astronomy with the discovery of transients defying our understanding of how stars explode. These extremes of the transient paradigm represent the brightest-called superluminous supernovae-and the fastest-known as fast blue optical transients-of the transient zoo. The number discovered and information gained per event have witnessed an exponential growth that has benefited observational and theoretical studies. The collected data and the understanding of such events have surpassed any initial expectation and opened up a future exploding with potential, spanning from novel tools of high-redshift cosmological investigation to new insights into the final stages of massive stars. Here, the observational properties of extreme supernovae are reviewed and put in the context of their physics, possible progenitor scenarios and explosion mechanisms.