The Tavern

Fulm Prism is one of my favorite spells from a starter book. I think it's very strong in trunk.

I once won a Djinn Conjurer who didn't get any decent spells before spell level 6 or 7. Without Fulm Prism that run would have been far more painful.

There's even a use of Prism the OP didn't mention: it can block hexes very reliably. Most useful in Slime for the mutating eyes.

I like that it has a fixed range - you know exactly what tile gets hit and what doesn't - which is very different from many other area-of-effect spells.