Eta Carinae is the most massive and most luminous evolved star known in the Local Group, and it provides a glimpse of the violent phases of eruptive mass loss that can occur in unstable massive stars before they die. Despite a wealth of high-quality data that help constrain physical parameters of its major eruption seen in the 19th century, there has always been a fundamental mystery about the cause of that event. The recent discovery of light echoes from the 19th century eruption allow us to obtain a time-sequence of spectra for an event that occurred before the astronomical spectrograph was invented. This allows a meaningful comparison with eruptive transients currently being studied in nearby galaxies, and also provides a link to the mass, kinetic energy, and geometry of the eruption that we get from the bipolar nebula around Eta Carinae. Most interestingly, light echoes reveal decisive evidence that the eruption was an explosive, shock-powered event, and the combination of available clues point toward a massive star merger in an evolved triple system as the cause of the eruption. This will be discussed in the broader context of massive-star evolution, the diversity of non-supernova transients, and pre-supernova mass loss.