Agent Apocalypse got his name by being the spark that destroyed an entire planet. He got his career by being good at what he does.
Habitable planets aren't all made the same way. The planet that Apocalypse was from was surrounded in a gas envelope of a mixture containing highly combustible oxygen gas. Over the course of millions of years, the combustibility of the outer atmosphere would slowly increase. One day the combustibility of the outer atmosphere would be so great that the slightest trigger would ignite the atmosphere turning the entire planet into a giant fireball. Then, the cycle would repeat itself, the atmosphere would begin to build up again, taking a few million years to reach fireball combustibility. Sometimes life got the chance to evolve to intelligent levels; it was such a period of time when Apocalypse was born.
As a small boy, agent apocalypse managed to stow away on a space-bound freighter leaving the planet. When he was discovered as a stow-away, the captain of the ship who was somewhat cruel decided to ‘throw him overboard’. Apocalypse took a deep breath before being shoved out the air-lock of the space-freighter. He couldn’t hold his breath against the vacuum of space, so he decided to scream as loud as he could and die quickly.
Unfortunately, Apocalypse’s scream of death had just the right sonic characteristics to ignite the oxygen surrounding the planet. A giant fireball engulfed the entire planet, as had happened many times before. That day four trillion people on the highly habited planet died. It was recorded in galactic records as one of the largest accidents ever to occur.
Amazingly Agent Apocalypse was one of the few survivors from the planet. A nearby Starfleet vessel saw what happened to the little boy and transported him to their vessel just in time to save his life.
Star fleet decided to turn the little boy into a spy – code named Apocalypse.
Agent Apocalypse's specialty was dying worlds. Whenever the 'end of the world' was eminent, agent Apocalypse would be sent in like the angel of death. Apocalypse's mandate was to assist in the migration of people to a new world, and assist in the stabilization of the people who could not be saved. Not everybody makes it off a dying world before it's too late. Star-fleet puts in a 'best effort' but the best effort isn't always good enough. Sometimes there just aren't enough resources or enough time to save everybody.
In one case Apocalypse helped organize the construction of 35,000 starships over a ten year period using up the resources of the planet as fast as they could. They were able to save about 35,000,000 people (One percent of the population). Each starship could transport about 1,000 people. They kept constructing and launching starships as fast as they could, right up until the end. At first nobody understood what was going on, or what was about to happen to their planet. It was an unpredictable event, something that gave people a matter of only years to prepare, rather than the usual galactic time-scale. The tricky part was being able to use vast resources to setup a factory system without anybody finding out why, in order to avoid a mass panic. There were all kinds of protests about companies using up natural resources, until the day it was made public that the 'end of the world' was at hand. People got off the planet basically by a lottery system. Winning the lottery got you a seat on a starship.
The situation is always different, and the solutions never the same. In another case they were able to fly an entire planet out of harms way, but there was a catch. They had to freeze the entire population of the planet. Large high-powered warp drive engines were mounted all over the planet at strategic locations. In this case they had many years to prepare to move the planet. It was considered a success after the planet was moved to a new, safer location. About 90% of the planet's population survived the move.
In yet another case, 4.5 billion hover cars capable of surviving an interplanetary flight were constructed. The species they were built for was not a social one, once again the populace was kept ignorant of the fact that the hover cars could travel in space. There were many complaints about the sophistication and expense of the hover cars until the day they were needed. People thought the navigation controls were overly complex, and the engine power unnecessarily powerful.