Fiddlesticks is one of the best champs to have in a team fight, and can be a real game changer. If used correctly, he can really dish out some hurt on the other team. This guide will teach you how to use fiddle in early, mid, and late game.

Pros
Great nuke
Easy to lane
Good survivablilty

Cons
Squishy
Terrible in 1v1 encounter
Bad farmer

standard caster runes, except i use armor seals because he only starts with 8 armor, which is rididulously low (most ap carries start with between 10-20 armor).

standard ap masteries, no surprises here.

Ghost and Flash are the best summoner spells to have on fiddle. They provide quick escapes when needed, and can be used to catch up to enemies. Being in range is important for fiddle. After using Crowstorm they can easily get away. Remember that you can still use spells and abilities during his ult, so ulting and flashing is one of the best combos that fiddle has.

Sapphire crystal
A good starting item on anyone. Fiddle is great at kiting and abusing enemies in lane, but to do this you must have mana. I rarely use the health pot, but it can be useful if you are getting outzoned.
Mejai's soulstealer
An easy to use item on fiddle. He deals great dps with Bountiful Harvest, and his ult is almost guarentees a kill. Get this item asap to have as much time as possible to get stacks
[Catalyst the Protector size=32] Catalyst the Protector
Complete the sapphire crystal into catalyst to give more survivability. I like to keep catalyst as long as possible to get the most benefit from the passive.
Sorcerer's Shoes
Standard ap boots, stack nicely with his passive
Rabadon's deathcap
most ap from a single item. I get this early because its important for a nuke to do as much damage as possible.
Zhonya's Hourglass
After getting Rabadon's Deathcap, this item will get you 130 ap. The passive is great for staying viable during your ult. Even during your ult, enemies can still kill you. A stun will stop your Bountiful Harvest, and leave you very vulnerable.
Void Staff
In my opinion, the second best ap item in the game. With Rabadon's Deathcap it gives 91 ap, and the 40% mag pen is really important. With this, even the tanks on the other team will fall to your ult. 30 mag resist is subtracted from innate and Sorcerer's Shoes, then 55% from this and your masteries, which leaves stacking mag resist almost useless.
Banshee's Veil
Time to complete your catalyst the protector. After your Zhonya's Hourglass is used, you still need something to stop stuns and knock-ups. It also gives great mag resist, which makes him more tanky than usual.

Amplifying Tome
If you dont like kiting a lot early game, then this would be a better item to Sapphire Crystal.
Morello's Evil Tome
If you aren't confident in getting stacks with Mejai's Soulstealer, then this is a useful replacement. the cooldown reduction works great on fiddle, and lets you ult more.
Rod of Ages
If you want more health or ap, then substitute this in.
Rylai's Crystal Scepter
This also gives a lot of health, and a slowing effect. This can help your team by providing more crowd control.

Terrify is one of the best crowd control effects in the game. It lasts for 3 seconds after maxed out, which is basically a 3 second stun. Bountiful Harvest is a good source of dps and healing, and fiddle's main source of damage output. Reap provides a silence and damage with each bounce.
I like to alternate on maxing out fear and Bountiful Harvest. Many players like to max out Reap, but i find that useless. Reap is only useful to farm, and most likely won't help in team fights. fear imobilizes an enemy, and using Bountiful Harvest right after provides more damage than from Reap.

Fiddle is a terrible farmer. Because of how low his ad is, the lane tends to get pushed. I like to hit creeps that are near full, then run around until a creep is ready to be last hitted. Don't waste Bountiful Harvest on a creep unless you're low and need health, or have plenty of mana.

Because of the long range on Reap, I like to abuse enemies with it as much as possible. Using Terrify then Bountiful Harvest can also do a huge chunk of damage. After reaching level 6, Crowstorm should be able to wipe out any enemy at almost full health. Make sure that they don't have an escape, however, like vlad's pool or tristana's jump.

At this point, you should be trying to get blue buff as much as possible. It's important to have blue buff before you Crowstorm, so that there is less cooldown. Sometimes, you have to save Terrify for when an enemy ults. Fear will successfully stop Katarina, Miss Fortune, Nunu, and many more.

Fiddlesticks has the greatest game changing ult. Using it right can lead to a victory, and can easily give a triple kill. Using this build won't guarentee victory, but it does give great damage and survivability. So yeah, get ready to own, and i hope you enjoyed my 3rd build