Alesta Seelie is an ancient Falmer.
The last priestess of Auri-El, unfrozen by the Dragonborn's adventures at the mountain temple dedicated to the god's cult.
 

She was frozen in there from a time long before the temple's downfall. As either a penance or sacrifice, she was set to remain in slumber until the time came. Except it never did. The entire Falmer race was betrayed and became something hideous, a shadow of the once gracious beings they once were.

And so, the priestess remained undisturbed and forgotten by many eras.

By the time the Dragonborn and her companions found her, she was disoriented and without a purpose.

The strangers that had awakened her offered her assistance and kindness. She remained with them for a while, in which she learnt to adapt to this new era and its customs.
It was not easy but with time and perseverance, she was finally able to travel on her own.

A side effect of her slumber was the uninterrupted growth of her magical connection.

This resulted in her newfound ability to cast spells at least 20 times stronger than even the strongest human Archmages.

The older some elves get, the stronger their magic affinity grows, and this particular elf, was old, and strong enough to unravel the weaves of Aetherius with a thought, or at least that's what the mages at the College of Winterhold concluded after testing her.
Leaving that place, she noticed they all looked at her with a hint of fear in their eyes. She meant them no harm, but there was nothing in that place for her either.

And so the elf found herself wandering in the frozen planes north of Skyrim...

 I thought... I was the last one...

We have much to discuss...