Can a Pyromancer Sorcerer deal fire damage to a creature that is under the effect of Invulnerability?
The spell Invulnerability (XGtE) states:
You are immune to all damage until the spell ends.
The class feature Fiery Soul (Plane Shift – Kaladesh) states:
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.
My gut reaction is no, because Invulnerability states you are immune to "all damage", not "fire damage" or "all damage types".
Best Answer
This requires a DM ruling.
As quoted in the question, the Invulnerability spell grants immunity to all damages, including hence all the types listed here and moreover grants immunity to magical and non magical damage, by a strict reading of the description.
Fire damage can come from both magical effects (e.g., Fireball) and from nonmagical effects (e.g., a Dragon's breath, ). Hence, what happens when a creature under the effects of the invulnerability spell is hit by a spell cast by a Pyromancer Sorcerer that deal fire damage? There are two possibilities:
The Fiery Soul ability treats the immunity from fire damage as resistance to fire damage, but the immunity from magical damage still holds, then no damage is dealt.
The Fiery Soul ability treats the immunity from fire damage as resistance to fire damage, disregarding the immunity from magical damage.
Here it comes the need of a DM's call: they have to decide which one of the above should be applied. Indeed, one may decide to loosely read the description of the spell, without making the distinction between magical and non magical damage.
As a DM, I would apply the first ruling: all damage includes both magical and non magical sources of damage, a spell dealing fire damage is actually dealing magical fire damage, and Invulnerability grants immunity from this kind of damage.