Fiddlesticks, the harbinger of doom is mostly used as an ap carry in the middle lane. However, with this build, Fiddlesticks can become an inconspicuous tank, attracting all of the damage like a universal taunt, like tank yi. His drain is extremely important to sustain himself during massive teamfights. This is why you start building ap and then you sink into tank specs. During laning you will be able to sustain for such a long time due to your masteries and your runes. Make sure you have a CV in your team so you can go against two champs who are AP.

For runes, I take all magic resist runes. This is because you don't have to worry about magic resist early on. You need as much money as you can early game.

For masteries, I go a standard 9/0/21 making sure to get Ardor, thus increasing your ability power early.

First item you get is boots and 3 pots. This is for more mobility around the lane so you can run in and out, throwing Reap all around the place. Just because Fiddle will be a tank, doesn't mean not to invest in a little bit of AP. Also you need LOTS of money, so you need Kage's lucky pick. Then you start to build tanky. Get a Warden's Mail and a Heart of Gold, thus giving you some good health and some armor.

Now leave those two GP5 to rake in some good gold, then start building a Thornmail if they're AS heavy. If not, then you could start building FoN and switch them in the build order. Now you will need to finish that Randuin's Omen. Now if you look at your MR, you shall notice it's kind of low if you indeed built the Thornmail. Now you should build the FoN. Now for those good assists, you will need Rylai's Crystal Scepter, so your Caw Caw Caw can slow your enemies as well as dealing some cool damage.

I guess you could also use a Spirit Visage, seeing as it buffs your drain to almost pre-nerf regen.

In conclusion, Fiddle may be used as an AP carry, but it seems like his true place is as a support tank. Drain restores too muh health not to exploit. With this build, you will be both saving and assisting your other allies in battle. Don't go the sure way, go Garensticks.