Millions of years ago, the entities known as the Celestials performed genetic experiments on the reptilian ancestors of the Skrulls, resulting in three subspecies; one with an unstable genome and shapeshifting abilities, one which was composed of immortals at the peak of their genetic potential, and one which had implanted inside them a latent gene series that would later promote rapid mutations. They warred with one another. The shapeshifters were triumphant, and after wiping out all other Skrulls began to expand their territory. They would later split into two more groups, one being the modern Skrulls and the other being called the Dire Wraiths, a parasitic race that could shapeshift, drain the life essence and memories out of its victims. They could use magic, but were not as technologically advanced as the Skrulls. And so the Skrulls forced them to flee the Skrull domain. They were eventually exterminated in the late 20th century.
In their interstellar age, the Skrulls evolved into a peaceful civilization, primarily interested in free trade and willing to share their technology with all races they deemed worthy. Their shape-shifting powers helped them foster bonds with their trading partners.
About one million years ago, a Skrull delegation came to the planet Hala in the Bama system in the Greater Magellanic Cloud, home to the then barbaric Kree humanoids and the peaceful Cotati plantoids. They held a contest between the two races to determine with whom the Skrulls would trade. The Cotati won, but the vengeful Kree massacred the Skrull delegation, took over the Skrull landing spaceship by force, and developed their own technology from it with which they began waging war against the Skrulls, decimating ten of their worlds. The Kree aggression molded the Skrulls into a ruthless militaristic civilization bent on eradicating their aggressor. Their war has continued on and off to this day. It is possibly this war that cristallised the skrulls in their modern social form.
Modern Skrulls are highly advanced sexuate sentient shapeshifters of reptilian origin. In their social form, they appear like green skinned humanoids the same height and weight as Kree (and humans), with large pointed ears and corrugated chins. They are able to assume virtually any form, be it organic or inorganic, adjusting there density to fit larger or smaller volumes. As a result, the Skrulls excel at spying and infiltration. Skrulls are also able to use their shapeshifting abilities to form weapons (eg. blades and clubs) with parts of their bodies, making them dangerous hand-to-hand combatants.
Despite the fact that young Skrulls hatch from eggs, mothers nurse their children to provide nourishment. They live an average of 210 years.
A hundred thousands years ago, Skrull scientists created the first Cosmic Cube, a device which could transform thoughts into reality. The power hungry emperor used it to expand his personal influence and holdings throughout the empire, but in time, the Cube developed a consciousness of its own. It patterned its personality on that of its user and destroyed two-thirds of the Skrull empire before realizing it had made a mistake. After time and hard work, the empire of the Skrulls was restored. It is at this time that the Skrull Throneworld was moved from Skrullos to the more central Tarnax IV.
During the second half of the 20th century, possibly after detecting Kree activites on the planet and realizing its population had also been subject to Celestial manipulations, the Skrull Empire showed interest in conquering Earth.
In 1984, After his Herald Nova annihilates the Skrull fleet, Tarnax was devoured by the Galactus entity.This plunged the Skrull empire in a power vaccum as the fight between five warlords for leadership threw their civilisation into a open civil war, which eventually lead to a reopening of the hostilities on the part of the Kree. Once a new empress gained access to the throne, negotiation of a truce with the Kree after defeating them in a decisive battle brought back peace. She quickly restered the empire and reesablished the throneworld on a planet named Satriana.
In 2007, The Skrull Empire, barely back on its feet, was also the first of the major interstellar empires to be hit by the Annihilation Wave, an highly advanced and aggressive outer-dimensionnal communal insectoid species that laid waste to much of the Empire, destroying dozens of Skrull planets. The Skrulls are now attempting to convince a Skrull-Kree hybrid, to become the new Emperor.
Source : Marvel Universe. First appearance : Fantastic Four vol. 1, #2 (Jan. 1962). The Skrulls were created by Stan Lee and Jack Kirby.