Who And What Are Marvel's Eternals?

While Phase 4 of the MCU is still largely shrouded in mystery and speculation, there has been a steady smattering of casting news revolving around one specific upcoming project: The Eternals, which so far, has reportedly nabbed big names like Angelina Jolie and Kumail Nanjiani as well as (potentially debunked) rumors of Stranger Things' Millie Bobby Brown as unconfirmed roles in the ensemble.But major talent aside, the question still remains: Who--and, perhaps more importantly, what--are Marvel