Of course, Kossuth would not do anything solely for an ally.
Without sufficient benefits, allies meant nothing.
The same held true even for allies of the Good Alignment... helping each other without shared interests was called making friends.
However, among the four major elemental gods, Kossuth was the only one deeply interested in the Prime Material Plane, even willing to answer certain grand rituals performed by his followers.
Therefore, he would inevitably be interested in a world that had been cut off from the Elemental Realm for many years.
He truly enjoyed developing new territories.
Kossuth possessed a mentality born of the will of fire: roughly speaking, he wanted to burn the entire world.
Just like flames that, given the slightest chance, would never spare even an inch of space and would burn without restraint... rising from dead ashes only proved his nature.
So, if Asmodeus chose to open this world to the fire elemental god and allowed him to extend his hand into it, Kossuth would probably be very interested in coming to this elementally barren world.
Since the two allies shared a common goal, they would probably hit it off immediately.
These two also had one thing in common: they were both highly capable of taking action.
After all, although he had done a poor job of it, and his character was even less worth mentioning, Amaunator had been the first among Toril's gods to deeply involve himself in the human world.
While the other gods merely responded to their followers' prayers and shared their joys, anger, sorrows, and delights with them, Amaunator had already begun managing his followers and devising sets of rules to restrain their behavior.
Setting aside whether he had done it well, he truly was the first to do so.
And in the years