When supernova ejecta racing away from their origin at over 10,000 kilometers per second slam into a wall of circumstellar mass, the resulting light show is spectacular. X-rays and radio waves zip off electrons that compressed magnetic fields have accelerated to the speed of light, while the gas that has been heated to more than a hundred times the sun's surface temperature outshines even the supernova itself. Like reading a diary, as the interaction continues we see the very last years of the star's life – one of the only probes of the very latest stages of stellar evolution. In my research, I model these extreme systems and calculate observable quantities to compare with what nature is producing and to inform observational campaigns for finding these events and the stellar evolution models that seek to explain them.