you don't explain... but i like it :)

I don't know why you have so much mana in runes when you get an AA staff, I think there are moe interesting choices on runes than mana... like magic penetration, which is not that easy to get... Or for instance CDR. For a loss of 6 percent CDR and 100 mana, you can gain 20 magic penetration

(isntead of CDR boots, i get mpen boots - -15%CDR and +20Mpen, and instead mana glyphs i get cdr/lvl glyphs - -100 max mana and +9% CDR )

... I really like magic penetration :)

I like that you maxed out CDR in this build, or well almost, one elixir of brilliance is all it takes...
- even with the changes above, you would be able to do that, since the elixir gives 10%

I do not know what to think of the warmogs. It's a strong item, but it gives only health and health regen... you get zero magic resist, and therefore, you still burn very quickly, even with 3k health.

You have two items for mana regen - AA and DFG. honestly, i do not think fiddle needs a DFG... it's another spell, and fiddle has enough of those. I think a spirit visage would compliment fiddle way more. You loose a bit of CDR, but now you get extra health and magic resistance, and you buff your drain at the same time.
- yes, it is a buff, your healing gets buffed by 10 per second, which is not a lot, but with the MR, you actually loose the health slower now

Now you might say that you lost a lof of damage by taking SV instead of DFG. You also gained survivability. So you might get a rylais crystal scepter instead of the warmogs, there you get more ap -> more healing, you loose a lot of health (about 500) and you gain a slow for your drain, your ult, and your crows.

Now imagine a fiddle who ults, and it's this kind of fiddle. His ult hurts, but most fiddles are paper, so let's focus him down and win! But no, it's a tank, so let's run instead! now you pop your randuins, and together with the slow from rylias, your enemies will feel like they stepped into a quagmire...

Your build is good, but lacks a lot of magic resistance, and every team should be able to have enough magic damage to take you down with 30 magic resistance. Now spirit visage might not be quite enough, but it is defnitely more than nothing... if you had trouble with magic, i would probably switch AA for abyssal scepter, or banshee's veil, but I feel that is very situational.

Your skill order is a bit wierd to me. You get fear very late. I normally get fear to level 3, to get the duration to 2 seconds. that is very useful, you can fear the one person who could interrupt your ult, then ult, because 2 seconds is how long the channel takes. also very useful if you want to use the ult 1v1. on the other hand the silence does not get longer, only stronger, and i do not see why i would need that. i normally get all skills on lvl 3, the order is very match dependant. then i get the fear to lvl 3 for the combo with the ult, then i max drain. after that i max fear and then i finally max the silence.

I have a few things to say about your masteries as well. You take ghost and flash as summoners. that is a very good idea. But you do not pick up the amsteries for them. Why? Also, Good Hands is defnitely better than preservance. Maybe you are a pro player and you never die, but honestly, the 10% are a lot when it stacks up a bit... in late game, it can save you a tower or two...

Now I don't want to claim that my way is better, I m just sharing my opinions :) The main reason why I even post here is the randuins omen, it's the first build that i found with the omen, after i independently rediscovered this amazing item on fiddle