Massive damage on slowed targets.
Empowered abilities and resource manipulation
Hitting a champion with spells hurls a comet that deals adaptive damage at them. Landing more spells reduces Arcane Comet's cooldown.
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.
Your first ability hit every 20s burns champions.
+20 Ability Power or +12 Attack Damage, Adaptive
You get free boots at 10 min. but you cannot buy boots before then. These boots give additional MS. For each takedown you acquire the boots 30s sooner.
+5% Max CDR
+5% Summoner Spell CDR
+5% Item CDR
Brand is a champion that can quite regularly land Comet once you have Rylai's.
The DoT from his passive also helps dramatically reduce the cooldown of Comet while they are burning.
As a high damage support, your job is to provide lane pressure with frequent harass from ability spam. Manaflow provides you the mana sustain to keep the pressure up.
Pay attention to the cooldown and try to utilize the mana refund on [W], your highest costing ability.
Celerity's Bonus Movement Speed is great for allowing you to skirt the edges of your ability ranges for lane poke and harass.
Scorch is a rune that really plays up on heavy lane aggresison and dies off over the course of the game.
If you expect to game to go to late you can forgo that early damage for scaling with Gathering Storm instead.
Magical Footwear allows you to take advantage of your aggressive laning phase damage by snowballing any kills/assists you pick up into a father damage itemization.
Cosmic Insight's well-rounded utility provides you more damage in the form of CDR and kill potential with Summoner CDR.