Kiting Movement Speed and late game scaling.
Empowered abilities and resource manipulation
Hitting an enemy champion with 3 attacks or separate abilities grants a burst of MS.
Periodically your next ability used has its mana or energy cost refunded and restores some of your missing mana or energy.
Gain 4% extra MS. Gain extra AP or AD, based on your bonus MS, Adaptive.
Gain increasing amounts of AD or AP, Adaptive over the course of the game.
+20 Ability Power or +12 Attack Damage, Adaptive
After gaining a level-up or takedown your abilities will have their mana costs refunded for a brief time.
Deal 7% more damage to low health champions. Gain AD/AP after kills or assists.
Cassiopeia massively benefits from the bonus Movement Speed on Phase Rush so she can chase down her target and continue spamming [E].
Manaflow Band synergizes well with Tear allowing you to use your abilities more liberally in the early game for trades.
Celerity double dips for Cass. She can use the Movement Speed to position for more [E]s and Phase Rush procs will grant you more AP.
Gathering Storm's scaling AP makes you an insane threat heading into the mid to late game.
Cassio's natural spam-heavy kit with [E] allows her you take aggressive trades when you level up and continue to sustain mana.
Coup De Grace lets you burn down any target and finish them off ASAP.