the thing you want to do with your runes, is to get as much magic penantration as possible. this is main thing for fiddlesticks, as his lifedrain will get a lot more effective.
Also ability power is a nice thing to start with, with dorans ring, you will have a good chance of First blood.

When you want to initiate a team fight, and migt also want get away alive, it can be nice with some hp regen. and thats why ive put strengt of spirit into the build. also it gives a bit armor and magic resist, wich will be helpfull when you activate crowstorm. also the 4% more magic damage will get usefull, with the spell vamp and crowstorm together.

This is the point where many people dont really know what i want to do with the build. well, the thing i like with fiddlesticks is his ultimate. wich makes me enable to initiate the team fight. and break the silence. Sorcerer's shoes, is for the magic penentration, thats one of the things you'll need as fiddlesticks 1v1. but since this build is team fight initiator, it doesnt do that much.
abyssal scepter, does also increase your damage output, with fiddlesticks passive "Dread", so we now reduce nearby champions magic resist by 30.
Rod of ages will increase your health, wich makes it harder to get you down, with suddenly appering in the middle of five enemies.
Rabadons death cap is mostly in this build to do some damage also, one of the things i like, is when you have the chance of meeting enemy champions, and beat them up. thats the good thing about rabadons.
Zhonya's hourglass is actually the most important part of this build, its a most have item for fiddle. now heres the reason: when you apear as the first person in a team fight, all the other teams champions will focus on you. then you just activate Zhonya's hourglass, and you will dmg them all, while beeing protected.
Will of the ancients also makes you harder to get killed, when you are dmging 500 dmg/sec, and getting 125 hp for each champion it hits.

When you and your team are getting ready to go, you are the first who are joining the fight.
The team needs to be ready, because, as soon you are in, you will live for 5sec, with no help, so getting all the help you can, is important. after the Zhonya's hourglass effect exspires, you are gonna have to try stay alive, wich isnt any problem with fiddlesticks. The only thing you have to do, is to cast Dark wind, and after that just life drain, like if it counted your life.