This is my build for fiddles. The reason why I get terrify to level 4 is so that you can just one on one anyone by terrifying then using your ult. With lichbane you should be doing plenty of damage after using your skills and terrify will be your chance to escape. Zhonya's ring will save your life plenty of times and void staff will give you great ability power + penetration.

Skil Braekdown
Terrify- Terrify- A great CC and crucial in team fights. This is what may decide a kill or maybe a save. It will stop enemy in the tracks buying time for you to escape.

Drain-Drain is a good skill because if you are fighting an enemy with no channel breakers, late game you should be able to drain enough health for you to take no damage. This is crucial in helping you survive in lanes and you can use it for a quick heal before a battle.
DarkWind- Fiddles Harassing skill. It bounces and silences. How useful is that in a team fight against 5 heroes? At max level it bounces 10 times so it will be able to hit at least each hero twice and silence them, which allows your team to take them down while they are silenced.

Skill Combo- My skill combo in team fights, late game would be, ultimate in, manual attack (because of lich bane) Dark wind, (manual attack) Terrify, life drain and manual attack. They should be dead after that. If not repeat, use zhonya to stall if your waiting for a skill to be back up. Just flash out and if you have max stacks + golem, your terrify and life drain should be back up in time to save your life and you should be able to finish them off.

In 1 vs 1, I would terrify, manual attack, ultimate, manual attack, dark wind, manual attack, FLASH if he runs, activate drain and then attack him once more. if you are about to die, you can pop in your zhonyas to buy some time until your terrify is back up/

Crowstorm-Fiddle's game changing skill. Late game you must not waste this, because this skill can turn things around easily. If your team is getting harassed and running you ult in and you dominate them causing your team to continue fighting. Since your ultimate will damage them like crazy, your team should be able to sweep them up. You can also use it as a finisher. So after using darkwind, your team starts fighting. You stay back and channel the ultimate. Boom you pop in, terrify their carry and take him down easy-peasy. After the main dps is gone your team will just mow down the rest.

Early Game
So you start off with a sapphire crystal and two health potions. In the lane your pretty squishy so don't try to fight too much unless you know you can kill them. You should be able to spam dark wind quite a lot and you should use it to last hit. After you have enough gold you should spam your skills a little bit and then back to get your catalyst. After getting your catalyst you should no longer be as squishy and are able to stay in lane longer.
Mid game
By now you should have a rod of ages and hopefully at least some boots of speed. You should be able to run around ganking people with your ultimate and terrifying them so they are unable to escape. After getting some assists or kills you should be able to afford your mejais. You should start some more major ganking and you should get golem whenever you can. After getting some kills you should be working on your lich bane.

Late Game
By now if you actually finish your lich bane, you should start on your zhonyas. It gives great ap and the active is great if you ultimate and just stasis. If you actually finish your zhonyas which rarely happens you should start making your void staff for the extra penetration. With some stacks you should be able to dominate team fights with your ultimate. If the game persists feel free to purchase elixer of brilliances.