+1 btw

Firstly, runes,
I think your CDR runes could be switched for some primary runes (like MP5(mana per 5 sec)), and the missing CDR could be added instead of flat AP. This would give you more CDR and a little more MP5 (since you dont get it in masteries)

Items,
I like the Haunting Guise at start for some MPen and MP5, but same as above, if you get MPen in runes, might as well focus on it in boots as well cause at this point you would get around 65 MPen, putting any enemy in negatives resistance.
After getting boots, I find Void Staff a bit early to really know if enemy is stacking MR, so I would problably go for a nuke item prior to Deathcap, like Abyssal Scepter (giving you a nice 85 MPen) or Morello's.
So the way I see it, the item build should be like this :
(1) Haunting Guise -> (2) MPen Boots -> (3) Morello's -> (4) Deathcap -> (If enemies not stacking MR) (5) Abyssal -> (If enemies stacking MR) (5) Void Staff

I must also point out that if enemies are harassing you very well or focusing you during team fights, you should get Rylai's at (3).

I'm gonna finish this section with a question : Have you really seen benefits of your spell vamp from Hextech? Cause in my tests, it never gave much and is another low item thats taking space and money for nothing (very personnal).

Skills,
I agree with starting with Dark Wind, but think you should choose from level 3 which one you will max first and max Fear with it. A good Fear with Flash and Drain deals a whole lot of damage without enemy possibly doing much.
This is pretty personal, so I'm not gonna bore you out with this section (I hope I'm not too late).

Summoner Spells,
Now I am totally with you, Flash is a no-brainer with Fiddle and Exhaust can give you the edge in chasing, 1v1 vs a dps, fleeing, keeping an enemy in ult or drain for 1 more tick, etc.

Now that should problably be most of it.

Cheers,

And I salute your efforts for the Build!

The mpen is taken care of with the void staff and runes, and depending on the map, the Boots of Mobility can be either very helpful or not too good.

Personally I usually go Rylai's a little earlier for the slow. I find it works wonders early on but doesn't help as much late game when I can kill without the slow mattering. Also I do agree that the sorceror's shoes would probably be better as the added magic pen will further increase your heal with drain.

But if you're escaping from a fight/chasing away from a fight, you won't get the full movement boost. That's why boots of mobility are mainly used by roamers, or people that move around a lot. Imo, sorceror's are better for the mpen.

Boots of Mobility are for escaping or chasing. And Fiddle really doesn't require survivability. His Drain does all the work. And I have never been in a single situation where I get focussed so bad I can't contribute. I'm debating switching the the Evil tome to something like Guardian Angel. Give the enemies hell when they waste their ults.

So. What should one do if you get focused, and can never contribute anything to the fight? You don't get any items that add survivability, and the boots of mobility seems out of place.