The Super-Skrull was the Skrull Emperor and also the oldest living Skrull (around one billion years old). When the Fantastic Four used a teleportation divice Reed built to travel a trillion miles to space they encountered the Skrulls. The Skrulls tricked them to take a so called cure for all disease and aging that also granted powers to anyone who used it back to earth. In reality it also included pico-bots that would dessicate their host when activated. Super-Skrull drew his powers from an advanced suit of armor that allowed him to mimic the superpowers of those around him whose metagenes had been activated by the Skrull pills. When most of Earth had used the medicine the Skrulls activated it and killed most of the population of Earth.
The Super-Skrull was killed when a non-super-powered Ben Grimm stripped him of his armor (with every other human on the planet dead, the Super-Skrull's suit no longer had any powers to duplicate). He was exposed to harmful toxins and bacteria in Earth's air before Grimm traveled back in time and erased the timeline where the Skrulls invaded Earth from existence by sabotaging the teleportation divice. This act also erased the Super-Skrull's death because he and the Skrulls now never traveled to Earth.