"You two clearly tread the paths of the swole. A rare divergence amongst your kind. Where are you hiding the protein?"
Ephrael Stern had been an ordinary Sororitas until an ill-fated mission to investigate an artifact, of which she was the sole survivor. The artifact contained a bound daemon and corrupted some of the sisters; those who weren't corrupted were flayed alive and bound together into a cage. But their faith in the Emperor was strong enough that they were able to imbue Stern with the knowledge and power needed to fight Chaos, allowing her to escape.
While still uncertain of what happened, Stern was found by her order and kept under quarantine, until she ripped a daemon out of one of her sisters and uncovered a Slaaneshi cult within the sect. This caught the attention of the Inquisition; one Inquisitor brought her to defeat the daemon that killed her fellow sisters, but was killed in the process. Other inquisitors thought that she was corrupted herself. Stern had died twice in her adventures, only to mysteriously come back to life not long after.
The Harlequins decided that they needed Stern as a potent countermeasure to Chaos, and sent the Pariah to bring her to the Black Library. Because she was being hunted by the Inquisition, she agreed to come. However, they were both captured for a while by Ahzek Ahriman, who wanted to enter the library himself. They were just barely able to escape into the Webway.
Ephrael Stern has made it to the Black Library, but she's very displeased by the Harlequin's food. Ephrael Stern is apparently very fixated on gains and mocks the Eldar for their more slender figures. She barges in on Cegorach's jokes to loudly complain about the Xenos' hospitality before vanishing back into the library.
She later reappears in Episode 29 as a means of collateral for a deal made between Magnus the Red and Cegorach, the latter sending the both of them to a moon where Eldrad Ulthran is making preparations for the ritual to birth the god Ynnead.