After Trixie's humbling defeat and the recovery of a dangerous magical artefact, the Alicorn Amulet, Twilight develops curiousity towards magical artefacts of old similar to the Alicorn Amulet.
During her research, she saved the world with her friends several times, became a Princess, stopped a heartbroken unicorn from destroying Equestria's present by ruining Twilight's friendships, and befriended said unicorn. Of course, all those events meant Twilight didn't make a lot of progress.
Now, however, she focuses solely on her research, fueled by the fear of what might be should she fail to find anything. To make matters worse, strange dreams keep haunting her at night, dreams of what might happen if, and when, she fails, each one about each of her friends, about her family, adding to her fear.
With her fear pushing her, it's only a matter of time before she finds something. However, what she finds might just make her dreams a reality, or it might just be what she needs to save her friends and family from a fate worse than death.