It is a good protection for a mage's tower, for it envelopes its external walls whith a layer of prismatic wall. The wizard(s) who cast it can pass freely through it. It can be rendered invisible whith some 5th, 6th or 7th spell (I don't remember the name). It works as a prismatic sphere, whith it's strenghtness and its weakness.
Casting this spell drains a point of constitution from the caster.
Components : 7 gems (min 5000 gp each) of the 7 rainbow different colors.