Knick Knack's small antique shop is visited by an old blue unicorn. He makes a very expensive purchase to be sent to the Castle in Canterlot. He also has anoher thing to ask. A favor of sorts. He wants her to take something from him that he has had for a VERY long time . . .
(This tale was written before most of my canon was established and thus contains elements that conflict with the canon.)
Immortality for the mortal must be a heavy burden, unless one can share it