Can anyone explain what this spell does please?
The spell converts fire damage to ice damage.
It's quite simple.
Say you are a fire wizard and you are battling a fire enemy. So whenever you cast a fire spell on the fire enemy, the fire enemy will resist half of the damage. That's where fire prisms come in. Cast a fire prism on the enemy. Then when you cast another fire spell on the fire enemy, the fire prism will convert the fire damage given to the fire enemy into ice damage. Therefore, instead of the fire enemy resisting the damage, it will BOOST the damage, because the fire enemy boosts ice.
This is EXTREMELY useful against enemies that have the same element as yours.