The Evocii were a simple, humanoid species who originated on Evocar. This planet was later transformed into Nal Hutta, the capital world of the Hutt civilization.
Around 15,000 BBY, the Hutts journeyed from their original home, Varl, to Evocar. The Evocii were greatly impressed by the advanced technology the slug-like aliens possessed. Consequently, they began to trade portions of their planet for the Hutts' technology, eventually realizing the Hutts controlled nearly all of Evocar. Hutt palaces and various other structures were erected across Evocar, and the Evocii found themselves unable to prevent further expansion.
They sought help from the Galactic Republic, but the Republic's laws at the time favored the Hutts. Many Evocii were forced off Evocar by the Hutts and relocated to the planet's fifth moon, which later became known as Nar Shaddaa, or "The Smuggler's Moon". Evocar was then renamed Nal Hutta. The Hutts demolished the remaining Evocii architecture and enslaved the majority of the remaining native population. During the Old Republic era, a small number of Evocii who managed to avoid enslavement formed rebel groups on Nal Hutta. These groups aimed to disrupt the Hutts and their activities. Huttsbane, a renowned warrior and leader of one such group, was greatly feared by the Hutts. Ultimately, these rebels were defeated and eliminated. The Hutts then exploited the Evocii on Nar Shaddaa as slaves, forcing them to construct the spaceports and cities of Nal Hutta.
Once the cities on Nal Hutta were completed, the Evocii from the planet were transferred to Nar Shaddaa to aid the Evocii already there. By the time the moon's cities were finished, the majority of the Evocii had perished due to prolonged exposure to the hostile environment. The few survivors sought refuge in the lower levels of Nar Shaddaa, which led to genetic mutations.
As an unavoidable result of generations of inbreeding and mutation on Nar Shaddaa, the Evocii physically deteriorated into pale, frail humanoids who were more vulnerable to Force-users' mind control abilities. Many sentients, including the Hutts, mistakenly believed the Evocii had become extinct.
In 26 ABY, the Yuuzhan Vong devastated Nar Shaddaa with a devastating orbital bombardment, then introduced various bacteria into the undercity to eradicate the remaining inhabitants. It is presumed that the Evocii were truly wiped out during this event. However, the existence of Evocii living off-world since the Cold War period, and the fact that they were no longer enslaved by the Hutts, suggests otherwise.