While the Darach was pretending to be Jennifer, using her face, Jeff Davis explains that there was to be a real Jennifer Blake on Teen Wolf in Season 5.
Unproduced Season 5 Storyline
For an unproduced storyline planned for Season 5, Davis says the writers decided that the Darach could not just make a face out of thin air. She had to borrow one from a living person.
Through ritual human sacrifice, Jennifer Blake was able to achieve tremendous abilities including fully changing her appearance and some telekinesis. It was enough power to take on and defeat most of the Alpha Pack singlehandedly.