Heavy CDR and maximum engage potential.
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.
Shortly after using your Ultimate, gain a decaying Movement Speed increase that lasts for 3s and allows you to pass through units.
Gain 10% CDR when you reach level 10. Excess CDR becomes AP or AD, Adaptive.
Gain increasing amounts of AD or AP, Adaptive over the course of the game.
+20 Ability Power or +12 Attack Damage, Adaptive
Gain a free stopwatch. Stopwatch has a one time use Stasis active.
+5% Max CDR
+5% Summoner Spell CDR
+5% Item CDR
If you're able to cast [Q] on an enemy before, or shortly after, dealing damage the Movement Speed slow on the fear is enough to make the Comet undodgeable.
Nimbus Cloak on Fiddle is fantastic for massive teamfight engages, granting you a ton of movement speed to collapse on the enemy team.
Fiddle typically wants to start building in AP itemization as quickly as he can. With Transcendence your mid-game scales decently as you can start building full AP after completing Zhonya's.
Transcendence also synergizes well with The Ultimate Hat to keep your [R] on a short cooldown.
Gathering Storm is your late-game insurance policy as Fiddle's [R] has a reasonably good AP ratio and scales well to win your team teamfights.
If it looks like you might be able to snowball an early lead, take Scorch here instead.
Perfect Timing allows you the freedom of hard engages early with the safety of Stopwatch and an efficient mid-game with a cheaper buy on Zhonya's.
Cosmic Insight is an all-around solid rune on any champion in any position. The summoner CDR gives you a few more Flashes for engage and a number of more Smite opportunities.