Dark Onyx, a Unicorn Mare-turned-Alicorn, wakes up in Canterlot after being gone for ten years, without remembering a second of it. she is sent to Twilight and the Elements in Ponyville so Twilight can study the amnesia and attempt to help Onyx. she is housed by one Anna Logue, who is Doctor Whooves' Sister and also a Time Lord.
Old, and unknown, enemies resurface.
it just might take a Reaper and a Time Lord to get Equestria through some dark times