Creatures with variable resistence get to have resistance vs one damage type as a free action, a variable number of times per encounter. Since you can take free actions during others' turn, it used its power against the radiance of the prismatic burst.
Sorry to be a stickler Ata, but variable resistance cannot effect radiant damage - this is a big deal for Brudd - you can only choose from acid, cold, fire, lightening, or thunder. Page 282 Mounster Manual. I really hope that has not been erratad(sp?) as Brudd will befar less effective!
Ugh, I don't know how DMs keep track of everything. I forgot about the combat advantage from the blinded bad guys. Would it have made any difference in my ice storm attack?