the only way i wouldnt get lich bane is if i want to build surival first, so ill get zhonya's or ryljai's

i tend to go boots - rabadon - lich bane for items, in that order because i need speed, the ap % boost of rabadons, and the speed boost of lich bane, plus i get 80 ap with lich bane.

RoA is wrong! Dominion won't last enough to be useful if you don't buy it first in the game! I only build this on Leona and it's always my first item cuz I want the passive from catalyst. You should swap it for a Rylais! It's even worse cuz you stack 2 overtime items (Tear and RoA). Tears are ok cuz you only need tears to stack but RoA is 3k gold and you need the whole item to start stacking. If we consider you'll get it after 8-9 minutes playing (and more if you pass more time capping, defending), you wont be able to get the full passive of RoA.... you'll be weak until it stacked which mean, you'll have stacked it after the game when it's too late! It's not good for a sin! As I said, I only use this on Leona cuz I half tank/AP so I don't mind being the highest killer beginning of game, I just want to feed and protect my carry!

Also, I will only get Tears early! I would get boots first => tears => deathcap => morello => archangel (tears should be stack so will worth many than all other items) => void => rylais (maybe before if they pown you or replace with a zhony or abyssal if needed)

But actually, I would change the whole build cuz I agree with Craftistic. Assassin's need survivability items asap! I almost never buy a deathcap on Dominion. Too expensive and champs that need it dies too fast to be able to get it. I'll always go for def items w/ AP (abyssal, Zhonya) of full def items (frozen heart, thornmail, veil)... Your build is awesome to burst but if you die in 2 sec, you wont be useful. Will only work on 1 vs 1 and you have to initiate. If they initiate or are 2, you are dead (even if you kill one other one will go cap that tower that could have make your team won the game!)

This guide is not helpful. I play a lot of Lebanc in dominion and I'd have to say I disagree with everything here with the exception of some runes and the the first two items you buy.

The key to dominion is capping (of course) and LIVING. with this build you are a glass cannon that will be easily undone by MR.

One item that has saved my life countless times already with leblanc - Will of the Ancients.
Spell vamp is great on leblanc because she has virtually no armor/MR. Another item I get every game is an Abyssal Scepter. Lots of MR for you, minus for them. If there's high AD coming at you, get a frozen heart or a guardian angel.

The aim is not high AP in Dominion. This build will ONLY EVER work if your team is rolling and you have little resistance. Can't remember the last game I actually bought a deathcap...

Simpleday is right, but only concerning glyphs. You want ap/level in your glyphs as they surpass straight AP by level 6. Seals stay straight AP for max early game effectiveness.

One last thing, it doesn't look as though you understand Leblanc's ult. It mimic's her last ability +40% dmg (at 16). Given that Sigil of Silence is your killer most of the time, what you want is to have your Sigil ready as your ult when you enter a team fight (if it's not, hit a creep on the way if you can). With Sigil then as your Mimic, You LEAD with your ult (doing 40% inital dmg, THEN 40% more when the next ability hits PLUS a silence). See what I'm saying?

R-Q-W-E. If target is not dead (many times this will insta-gib), well you just bought yourself time by ending with E. **** it, I'm going to make a guide...

I think tear of goddess should be bought earlier, and boots can be Ionian Boots of Lucidity to get max stacks to tear of goddess

I voted plus, for your good work, but I disagree with many things.

Runes:
Why not /level runes? You will be level 8 really fast, and you lose a lot of ability with flat ap runes.

Masteries:

Perseverence maybe usless in dominion? You get potions and your starter items instead put on utility mastery and 1 point on expended minds, much better IMO.
Greed, again, useless for dominion.

Items: No magic pen... interesting.