Convert is just that it converts your myth damage spells into storm damage. We use them against another myth so instead of him resisting your spell you actually get a boost from it.
Enemies have resistance to their own schools, so Prism and Mass Prism spells are ways to get around it and deal extra damage to boot by targeting their weak school.
Myth enemies might have a 70% resistance to Myth spells, but by using a Prism, it turns in to a storm spell and does +15% extra damage now.
One thing to note though is, Traps and Prisms work on a Last In, First Out system. So if you put Myth Trap on an enemy, and then a Prism, the prism triggers first and makes the attack storm, leaving the Myth trap alone because the attack is now storm.
However, if you placed a storm trap first, then the prism, then a myth trap, all 3 would trigger in the proper order.
Ah, thank you all, I'm Death/Life with like, one myth spell and my family are Ice/Fire so, I'm guessing this is of limited use to me unless we have a ton of Myth spells or happen to team with someone that does.