Not Updated For Current Season
The masteries shown here are not yet updated for the current season, the guide author needs to set up the new masteries. As such, they will be different than the masteries you see in-game.
Here's my Fiddle build and how I play him, as well as something tips and observations on his abilities.
I always feel these are up to you. If you'd rather more magic pen than AP, get those. If you rather get some CD reduction, that's fine too. The Quints are the most arbitrary of all. Put what you feel Fiddle lacks the most in your game play in the Quints.
Always go x-x-21. you can do 9 offense or 9 defense based on your own opinion, but Fiddle is too squishy, so I go defense. Give him survivability so you can live to cast another spell.
3: Summoner Spells
These are also player preference. Flash is a must, as you should know from anyone who ever plays Fiddle. If you was do be sure a hero isn't going to escape your crowstorm, get either ghost or exhaust. I would recommend ghost, because you can chase multiple players instead of just slowing one. I, however, go with ignite. Because ignite kills. period.
4: Gameplay and Items
A lot of people like to go "jungle-sticks", but I find he is exceptionally deadly in a side lane or middle, so that is what this build starts off as. Get your ring and health potion and pick a lane.
Best Choice: solo mid
Fiddle can harass really well in the middle. drain is one of the most effective ways to keep the other hero away from your creeps, and dark wind's bounce makes it great for pushing and harassing. Against someone like a Miss Fortune, who has a lot of speed on you, you can aim the dark wind at a nearby creep and hit her without having to be in range.
Also good: Go top if blue, bottom if purple
Basically, the same strategy applies in terms of harassing, but you want to work with your partner. Hide in a bush (be careful, your passive can give you away) and Darkwind+drain, or darkwind-fear-drain right when your partner move in for the strike. or when the enemy moves in on them. The reason for the specific lanes is to get that pesky golem when you have a free minute. It helps a ton.
At level 6, the fun really starts. hide off to the side, or let your partner distract them. crowstorm when it looks like you can get between them and an escape. cast darkwind, then drain or fear as is needed. Done right, you ought to get at least the assists, or at best multy kills every time. If you went mid, be sure you gank side lanes. Do this in your lane till you kill a tower.
Mid Game and Late Game:
Once a tower's down, go to town. You should already have the soulstealer. Buy the mercury treads. No, here is where my items get iffy. People have told my i should just focus AP, get a tone of damage, but that's not really true. Fiddlesticks is EXCEPTIONALLY squishy, and his spells do not scale perfectly with AP, so you get diminishing returns. However, his spells have relatively low cooldown with the exception of crowstorm, and can be spammed. But only if you stay alive. After buying the treads, you need to make a choice: how are they trying to kill me?
Lots of ranged dps? Get randuins.
Lots of heavy nukers and stuns? Banshee Veil
Lots of both? Guardian Angel.
Those three will keep you alive long enough to put a BIG hurting on the enemy, if you pick the right one. Or if you can't pick or are lazy, pick GA. I actually prefer randuins because it is yet another slow at your disposal, and I tend to have armor issues, not magic resistance.
So, take your pick. crowstorm is your bread and butter. Land amongst the enemies and things just die. A lot. You can leap and darkwind if your friends are there to take advantage of the silence or you are in danger of knockback pushing you away. you can fear that one guy who has been killing your team and insure he's out of the way. or you could do both just for shiggles. But almost always lifesteal. keeps you alive, and alive is good.
The rest of the items are self explanatory, and if it gets to the point where you can get the lichbane? self doran's ring. You should be able to clean up and then push lanes no problem.
Good luck and remember to drain. Drain, drain, drain.