The Pyromancer sorcerous origin from Plane Shift: Kaladesh has a feature called Fiery Soul:
At 18th level, you gain immunity to fire damage. In addition, any spell or effect you create ignores resistance to fire damage and treats immunity to fire damage as resistance to fire damage.
This ability allows you to treat immunity to fire damage as resistance to fire damage… and allows you to ignore resistance to fire damage.
Does the "ignore resistance" portion of this feature allow you to also ignore immunity that is turned into resistance by this feature?
Note: This is not official D&D 5e source material. Rather, it is something like Wizards of the Coast's "official homebrew"; it's un-playtested material by James Wyatt.
Best Answer
No, it doesn't completely bypass immunity
When a feature is designed to ignore both immunity and resistance to a damage type, rather than "reduce/step down" one level of protection, it is quite explicit in this. One other class feature does this - the School of Evocation wizard's Overchannel feature:
Here, the ability is clear that it ignores both immunity and resistance because it groups them together. In contrast, the Pyromancer's Fiery Soul breaks each step down and separates them, indicating that it only works to either change immunity into resistance or change resistance into regular damage.