Here’s my version of The One Below All if they add him to the MCU
Instead of Gamma Radiation (which is a dumb concept TBH) he is associated with Chaos Magic.
He is the architect of Wanda Maximoff’s corruption as in the shadows, he is in a way "the author of all her pain” as he used her trauma and loss on her parents, her brother Pietro, her husband Vision’s deaths, and her need to find her children in order to corrupt her into making the Scarlet Witch free The One Below All so he can destroy the cycle of death and rebirth in the multiverse as her Chaos Magic is the only thing capable to destroy the multiverse and the cycle of death and rebirth.
He is based on the supervillain of the same name while his design is based on The Scarlet King from the SCP Foundation Mythos, Invictus from Final Space and Zombros Engar’s God of Terror form.