Anguis
The Twilight Snake.
Anguis measures at 20,358 miles long and is by several orders of magnitude the largest Arcea on record.
It's said she was visible from earth with the naked eye and that her body is comprised of billions of bleach white bodies.
Her awakening caused the immediate destruction of luna and directly led to the fall of earth.
Even among ancients she appears to be on a level all her own.
Every time she wakes the entire solar system is thrown into turmoil for years.