Magic Item Compedium, page 40. On crit the target gets to eat Prismatic Spray, even if it's normally immune to crits. Also, it costs +30,000 GP instead of having a bonus-equivalent cost, which is a little weird but is actually a pretty acceptable deal since it means it doesn't increase the cost of your other enchantments.
I used them for a dual-kukri Swiftblade critfisher, and decided to run with the theme, making him a sorcerer with prismatic-dragon heritage and picking up as many prismatic and rainbow spells as I could cram between the essential buffs.