Alastor is a Fallen Archangel theory
Maybe it was brought up many times before.
According to lore, Lucifer cannot harm sinners.
But in Season 1 he easily hit Alastor with his cane, just before "Hell's Greatest Dad":
Alastor clearly looks hurt here:
Therefore, Alastor cannot be a sinner. Which, by the way, opens a variety of opportunities, like travelling outside of Pride Ring.
We also know that the rules are black and white:
and if one is not a sinner - he's a winner, so Alastor must be a Fallen Angel at least.
Furthermore, during fight with Adam, Alastor clearly intended to end Adam's life:
In Season 2, Rosie mentioned that Adam was an Archangel. So, Alastor should be of no lower rank to really think to take on Adam.
My theory is, Alastor is Sera-level of power, but with a very specific taste and creativity. Possibly, limited by some unknown to us deal/s or punishment.