I've played Fiddle as my main for a year now and I usually am the one on our team to kick ***. I mostly play 3v3 as Fiddle works better on 3v3. The map have more brushes and nice places to ult through walls. I decided to publish my build as I win way more often than I lose and none else published a similair build. I'm not gonna post how to play Fiddle yet, but I'll post why this build works wonders.

Mark: Flat Magic Penetration (MP) [+8,5] - This works great on Fiddle because of his spells dealing lots of small hits, instead of big ones.

Seal: Flat AP [+5] - This will help you early game. You don't need extra mana with Fiddle when you have Clarity. He uses mana very slowly.

Glyph: LvL AP [+28 at LvL 18] - This will give a good ammount of extra AP at end game

Quintessence: Flat AP [+15] Extra help at early game. This can be replaced by Flat Magic Penetration if you don't like it.

In total: +20 AP at start - +28 AP at end - +8,5 MP at start

You might think "Why no CDR?". Well, as I play in 3v3, my ult is always ready for all the teamfights. If you don't think it's enough, just grab the green buff.

I take 9/0/21 - Colors indicate what tree its from.

Total Stats:

Basic stuff
+0,8 LvL AP
+9% CDR
+4% Regeneration
+5% Base Mana
+0,6 Mana per Sec
+3% speed

Awesome Stuff:
+15% Magic Penetration
+30% MonsterBuff Duration
+15% CDR on Summonerspells
and both Summonerspells upgraded.

Magic Penetration is one of the best masteries in the game and none should go without it.

Monsterbuff Duration can be really helpful, no matter if it's 3v3 or 5v5. Just remember to make use of it.

+15% CDR on Summonerspells will mean, that your ghost and clarity always, will be ready for when you need them.

Summonerspells upgraded will be explained under "Summoner Spells"

Now comes the interresting part.

I start out with an Amplifying Tome. +20 AP +20 from runes will give atleast 40 AP to start with, giving you a strong early game.

Then I get Sorcerer's Shoes as soon as I have enough money for it. The extra movement speed is always important, and the Magic penetration will give you around 28,5 in total, making your spells hit way harder.

Next I upgrade the tome into Will of the Ancients. Not only is it very cheap AP, but you'll also boost your teammates and the 25% spellvamp will boost your lvl 5 drain into 105% lifesteal, making you much harder to kill.

Next I get Abyssal Scepter. It'll give you both AP and good Magic Resistance, plus it reduces the enemy's MR. That means, you deal much more damage to enemies, (Including junglemonsters!), and they deal less damage to you. It stacks nicely with your passive.

Then I buy Rabadon's Deathcap bringing your AP up to 500+. A 3v3 match normally ends before I get it finish, but if I do, my spells suddenly damages WAY MORE. The dark wind will almost one-hit the normal minions by now.

Then, we are running dry on items since Rabadon is almost the last true item in the build. Get a hextech revolver. More AP, more spellvamp simply.

If the game draws even longer than that, build up to a rylai's. It isn't very good on fiddle, but we are running dry.

Final possible item, would be to upgrade to hextech gunblade, to give more AP and spellvamp, but i've never tried a game that long.

You might also try buying thornmail if they kill you with physical damage or voidstaff if you can't kill them.

I get Drain in max as fast as possible. Not only is it your main damage spell together with a well placed fear, but once you reach lvl 7 you can solo the dragon with it. (DO IT FOR YOUR TEAM!)

I take darkwind at lvl 2 to kill minions and poke champions. Otherwise i get ult when avalable and I try to balance out darkwind and fear, although fear is more important.

Please note that your ult can tele through walls to surprise targets. Always try to use it from a brush or behind a wall, where they can't see you. If they see you they will just run or interrupt you, so don't waste it like that.

Ghost will make it easy for you to both chase and flee. Don't be afraid to run. Only noobs fight a fight, they will die anyway. The upgrade both makes you faster and makes it last longer than others who have it.

Clarity will be one of your main sources to get mana. Because of Fiddle's slow mana useage, this will keep you in lane for much longer and will get the mana you need to get the health from drain you need, meaning more gold, and exp for you. I use the upgrade alot, and everytime my mana is low, I seek out my teammates to give them a refill too. (DON'T BE AN @SSHOLE! HELP YOUR TEAMMATES!)

I hope this build will help you just as much as it helps me. I can't think of any better way to max your damage output, while still giving good surviveabillity. Please comment below, what you think. Please note, that this build works in 5v5 too. I just wanted to share how it works in 3v3. If you play 5v5, you can solo bluebuff in lvl 3 (Drain 2).