Echo of Eons

Echo of Eons won't be put into your graveyard until after it's finished resolving, which means it won't be shuffled into your library as part of its own effect.

To determine the total cost of a spell, start with the mana cost or alternative cost (such as a flashback cost) you're paying, add any cost increases, then apply any cost reductions. The mana value of the spell is determined only by its mana cost, no matter what the total cost to cast the spell was.

A spell cast using flashback will always be exiled afterward, whether it resolves, is countered, or leaves the stack in some other way.