When Rachel Summers landed in the apocalyptic future ruled by Apocalypse, she knew she had to find a way to ensure that this future would never come to pass. Her plan to thwart Apocalypse's ascension dealt with protecting her brother, Cable, in the main timeline.
Protecting Cable was a priority because his powers should rival her own and he could stop the ascension and ensure a brighter future. In order to accomplish these lofty goals, Rachel sent someone back in time to protect baby Cable at a critical junction. The person's name was simply Askani, and she forsook her body to travel in back in time as a being of energy. The threat to Cable involved an attack by Apocalypse (as recounted in the Endgame story arc), but Askani was too late. Baby Cable was infected with a techno-organic virus, a terminal disease that had no cure in the present timeline.
To make up for her tardiness, and to enable Cable to still be the "Chosen One," Askani offered to take Cable to the future to cure him. This caused Cyclops, Cable's father, severe distress, especially since Askani told him that he'd never see his son again. Cyclops ultimately let his son go in hopes that the future could cure actually cure him. Cable would eventually be saved, but Askani would not survive the trip back to the future.
A Second Askani, a Second Trip
The Clan Askani once again sensed a threat to the Summers family when Cable's body was inhabited by Stryfe's mind. The Askani worried that Scott Summers and Jean Grey would be targets of Stryfe again (as was the case during the X-Cutioner's Song) and sent another member to protect the family. Again, the person called themselves Askani (though a different person) and, again, Askani had forsaken their body to travel as a being of energy.
Where Askani was simply too late the last mission, Askani this time wasn't even able to begin her mission, as Tyler Dayspring, the errant son of Cable, captured her energy signature and held her captive. His plan wasn't to allow Stryfe to kill Cyclops and Jean Grey, but rather to allow Stryfe to inhabit his father's body so as to exact revenge on both men at once. Capturing Askani was merely a way to make sure that nobody meddled in his plans. Askani's imprisonment, however, served as the catalyst to undoing Tyler's plans.
Tyler's mutant powers include the ability to tap into someone's psyche and project the emotions of the person's memories. Askani held the truth to Tyler's past, the revelation of which only further confused the man. Stryfe (in Cable's body) found the two shortly thereafter and obliterated Askani's energy form. Fortunately, Charles Xavier and Jean Grey were able to harness some of that energy and used it to talk sense into Stryfe by showing him the truth about his past. Once Stryfe released his hold on Cable, Askani's energy form supposedly dissipated resulting in a total death.