The Mass Effect series was filled with player choices and branching paths, so there are actually a few different ways for the Salarian scientist, Mordin Solus, to meet his end. In fact, it's even possible for him to fake his own death and survive the series, but in most playthroughs, he ends up dying in heartbreaking fashion in Mass Effect 3.
Mordin has finally managed to find a solution for the genophage, a virus that he had helped to construct which had caused the entire Krogan population to become sterile. He can finally put right his past mistakes and cure an entire species, but he has to sacrifice himself in order to do so.
It's such a tearful moment as we see one of the most adorable and endearing characters in the entire series performing a truly noble act in order to bow out with pride, rather than living in shame.
Players' hearts were broken when Mordin quietly starts singing his famous "I am the very model of a scientist Salarian" song to himself in the moments before his death.