Not Updated For Current Season
This is a pretty standard build on Fiddlesticks, but with slight modification.
Summoner Spells and Masteries
I'll start by explaining both the summoner spells and masteries:
Now, I don't think I'll really need a reason for why not to level terror or your ulti. Terror being your ultimate crowd control stops carries in their tracks during your ulti or to set up for a nice long drain. Maxxing out drain is completely situational. If you're getting harassed a lot, why not max it first, it'll give you some good regen withlittle mana cost, other wise I keep one point in it simpy because I'll only really use it if I need health or for small amounts of damage, otherwise, a good darkwind should be able to suffice double of what a drain can do.
Now, for the most part the items are again, purely situational. Being that fiddle is a very squishy champ (hero) I like rushing Rod of Ages due to it's high health, mana, and AP values, I believe it's a must. The boots depend on the enemy team, for the most part you'll want sorcerer boots for the magic pen. If the enemy team is trouble with their crowd control, perhaps mercury treads, otherwise sorcerer boots will do. Abyssal scepter I find extremely helpful. Being that it does stack with your passive, it'll significantly reduce the enemies MR (magic resist) (by 30, which is big early game, but big nonetheless). This'll improve the effectiveness on Fiddle's spells and perhaps other spellcasters on your team. Finally, I wrap the build up with banshee's veil, being that it voids CC, helps you cast your ulti, makes your health higher to a certain degree, and helps against casters. Though this build does not provide huge amounts of AP, it provides high health to stay in fights, quite a bit of AP, and makes you a semi-tank I suppose. Item like zhonyas are recommended late game if you are getting fed or perhaps late game after the core build. Mejai's is pretty good early game or mid if you're getting quite fed. Archangel's is perhaps decent if you need mana and a big AP boost. Rylai's is also good if you need a slow considering Fiddle's spells are almost all AoE, being able to slow an entire team is quite nice. Finally, void staff if the enemy team is reliant on MR. Again, these items are completely situational, really depends on your play style and enemy team.
These are setup for a basic caster. Insight so your spells do more damage and are better against tanks. Fortitude because who wouldn't benefit from health. Clarity because it helps with mana problems, which kind of defeats the purpose of Archangel's or clarity summoner spell. Finally, focus because his ult has quite a heft of time between usages, plus nobody could possibly not benefit from cool down reduction. This is a basic caster rune set imo, but feel free to recommend a better one.
Finally, I want to go over strategy. It's a no biggie early game, with you downing minions with darkwind, harassing enemies with your basic attack and what not, but when you hit level six, that's where it starts to get fun. Basically, what you're going to do is ult of a bush directly at a carry/ squishy (unless your team has already initiated on the enemy team, in which case you can go directly in for massive damage). Follow this ult up with your fear on the person your team is focusing most, or perhaps the carry. Immediately follow the fear up with darkwind to devastate a team with silences and then drain the feared target. In this way, you should be able to support your team greatly. If you misfire your ulti, use flash to readjust the positioning. With ignite slap it on the person you are going to cast drain on. Remember, ignite before drain. Chase down the enemy team with your ult on, and if you have zhonya's you can use that during your ult to be invulnerable to enemy fire as you devastate. Without your ult do the exact same combo without the two flashes/blinks and you should be able to team silence. With this strategy/build, preferably hunt down squishies (Ashe, Veigar, etc.) or Casters (Ryze, Annie, Veigar, etc with your silence). His ulti is great for ambushing, hence the title of the build.
THIS BUILD IS STILL A WORK IN PROGRESS, FEEL FREE TO COMMENT AND CRITICIZE, I'M OPEN TO IMPROVEMENT