Ultimecia is the main antagonist of Final Fantasy VIII. Because she operates through the body of a possessed Edea to gain control of Galbadia, Ultimecia's existence is revealed only after possessing Rinoa to release Sorceress Adel from her orbital prison to take as a new host. A sorceress from the future, Ultimecia is capable of reaching her consciousness into the distant past via a special "Junction Machine" to possess other sorceresses. She seeks to achieve "Time Compression", which would cause all eras to merge; this would extinguish all life but her own as she becomes an omnipresent goddess. This would give her power on a par to Hyne the Great, who, according to the background had created the world.
Source: Wikipedia, "[en.wikipedia.org/wiki/Characters_of_Final_Fantasy_VIII%23Ultimecia Characters of Final Fantasy VIII]", available under the CC-BY-SA License.