Cassiopeia is a very unique high-base damage, but low range and extremely item-dependant carry.
Her core item set revolves around giving her safety and maximizing her absurd damage output, Rapid Firecannon removes her short-range that may put her in harm's way, while Morellonomicon allows her to proc Blattecast's healing far more often and ignore Magic Resistance from enemies. Blue Buff covers all of her mana issues.
Active - Petrifying Gaze: Projects a cone of magic energy at her target, dealing 275 / 400 / 2000 (× AP) magic damage to all enemies hit, stunning them for 2.5 / 3 / 5 seconds, and causing them to take 10% (× AP) increased damage while stunned.
Active - Noxious Blast: Launches poison at the nearest unpoisoned enemy, causing them to take 700 / 1500 / 3000 (× AP) magic damage over 12 seconds, and reducing the effectiveness of shields placed on them by 50%.