Why is Lighterbringer the only item above? Because set build orders are dumb and I wanted to bring that fact to light. Build according to how your game is progressing, not to some dogmatic order. But here below is a general item analysis that helps me determine my build order from game to game.
Starting Long Sword
is a pretty safe start that gives you a head start on mid-tier AD items in exchange for a weaker laning phase. I think 3 pots is enough to survive until your first back but it is riskier against strong early game laners/junglers.
Starting Doran's Blade or Doran's Shield
is incredibly safe as both provide sustain against poke. If you think you'll be zoned from auto-attacking early game, go shield. Otherwise, blade is better overall item on Fiora.
Starting Double Rejuvenation Beads
means you are rushing Tiamat and intending push the lane as often as possible. This is best against champions that attempt to proxy farm or take advantage of the fact that Fiora has no natural waveclear. 10 hp/5 is equivalent to a potion every 75 seconds, pretty useful.
Starting Cloth Armor
is best against AD lane bullies like
. Make sure to use all of your potions, along with Fiora's passive to keep your health up.
Early Game Items
is necessary for sustain and later items such as Ravenous Hydra and Blade of the Ruined King. Season Three changes to its cost and formula has made lifesteal more expensive and rare. If possible, finish more expensive items first such as Tiamat or Brutalizer. These items are more helpful in lane since lifesteal is less useful without AD behind it. Besides you already have lifesteal if you took lifesteal quints as I recommended.
First Offensive Item
compensates for Fiora's complete lack of wave clear and provides hp/5 which is useful in lane, making it an ideal first item. Its active is essentially another AOE auto-attack on a short cooldown. The speed of this active is dependent on your attack speed so activating it during Fiora's E will lower the cast time. Try to weave the active in right after an auto-attack as it resets the auto-attack timer. So a combo would go like this: Auto + E reset + Auto + Tiamat reset. The active will not by affected by blinds, dodges or draw minion aggro.
is an excellent early game item for snowballing due to its Armor Pen and CDR. However it is not an answer to armor stacking, that's Last Whisper's job, but rather makes damage dealt to low armor targets more effective. I recommend holding on to the Brutalizer instead of upgrading it as Black Cleaver and Youmuu's are both very expensive and provide less AD then other options. Get Brutalizer against squishy targets, not armor stacking ones.
has a very nice build path and provides a useful slow to Fiora's cc-less kit. The range on the slow is only 500 so keep that in mind. If you require early game sustain badly to stay in lane or require cc, this is the way to go. Blade of the Ruined King has a poor build path that requires you to buy 2 daggers then another 1000 gold but its the best item to rush against pure tanks.
Middle Game Items
Core Offensive Items
Don't feel obligated to finish one or both of these items as soon as possible. Completing two mid tier items like Tiamat and Brutalizer will make you stronger in 1 v 1 squishy targets as opposed to rushing Hydra for a similar cost. The great thing about long swords is that they build into every core Fiora item so you can easily adapt to your situation. You should have both Hydra and LW by late game but it doesn't mean you have to rush them.
helps with Fiora's teamfighting potential so much and has such an easy build path that I'm moving into my core build. You don't have to buy it right after getting Tiamat as you may want more AD/Armor Pen instead of lifesteal but you should have it once 5 on 5 teamfights are frequent. Note, that the splash is calculated from total AD, not total damage so splash from Blade Waltz are not as powerful as expected. Because the splash also benefits from lifesteal, you want to use it around minions or other enemy champions for additional healing.
should either be your second or third completed item. Unlike MR, all champions (except Thresh) gain armor per level and even the squishiest champions will have around 80 armor late game with runes and masteries. Unlike most ranged AD carries that have low base damage and abilities, Fiora has such high AD and base damage on R that she should get Armor Penetration before she gets additional AD, lifesteal, AS or crit. Last Whisper is fairly cheap in comparison to other Armor Pen options and scales better into late game. Here's LoLWiki's take on it:
"Last Whisper becomes more effective at penetrating armor than The Brutalizer when an opponent has more than 28.6 armor. It becomes more effective than Youmuu's Ghostblade at 57.1 armor. It becomes more effective than 5 stacks of the Black Cleaver's debuff (+ flat pen) at 100 armor."
Other Offensive Options
Blade of the Ruined King
, from here on abbreviated to BORK, was a perfect item for Fiora until Riot reworked it. Now, it's gold value is a bit too lopsided towards AS and doesn't give any additional AD from Bilgewater Cutlass. The MS steal active is still great for both offense and defense. BORK counters HP stackers as it deals 5% percentage damage of their current health per auto-attack. Note that its 'current' health and not total, so its effectiveness will decrease as you damage them. Fiora makes great use of BORK's on-hit effect due to her E and her R's 5 applications of it. However I would not rush BORK unless the enemy laner is a tank, as BORK provides low AD. All of its damage is physical which means heavy armor stacking will still counter it.
The Black Cleaver
is more of a team oriented AD item that softens up enemy's armor for your AD carry. It has a poor build path after Brutalizer and isn't very effective as a first item. However as a late game item with Hydra, the splash will proc Black Cleaver's stacks, potential getting full stacks on the entire enemy team. Its not worth getting before late game. Especially good with Miss Fortune or Graves as allies.
gives every AD multiplicative stat: AD, AS, Crit, Armor Pen but isn't exceptional at any single one. As is, Youmuu's depends too much on its active to be cost efficient. However its still great for dueling and its active helps catch opponents with its MS. If you bought Brutalizer earlier and are focused on killing a single target late game, buy Youmuu's instead of Black Cleaver.
Getting boots early is less necessary due to the global MS buff to champions and nerfs to boots. That being said, they are still necessary around middle game.
Boots of Mobility
are such a great item for getting in range to lunge to a target. Because Fiora's E does not grant MS until she lunges, its necessary to have boots that can close the gap quickly. Since AD carries build Zeals, its necessary to move faster than them to get in range. Obviously this item is riskier since you don't gain any defensive stats from it but I like it for the additional movement to help assassinate carries.
isn't cost efficient, you only get 5 additional MR than a Null Magic Mantle however its the tenacity that counts. Fiora is very susceptible to cc so reducing the duration is helpful for surviving burst. However tenacity does not reduce the effects of knockups, knockbacks, suppression, and persistent AOE slows. Also tenacity stacks multiplicatively with cc reduction from masteries so it would be .65 x .85 = .5525, 1-.5525 = 44.75% cc reduction
instead of 50% cc reduction. The combine cost is higher at 450 gold so you'll probable have to buy it on the second trip back. If you need more straight MR, get two Null Magic Mantles instead of upgrading to Merc Treads.
scale well into late game because auto-attack damage reduction works multiplicative with armor and health. By late game most of the damage will be from ranged carries, so reducing auto-attack damage by 10% helps Fiora close the gap to kill them first. Its unknown how its percentage auto-attack damage reduction works in order with the block mastery and Doran's Shield.
is best when your team is losing and defending your base. Even if your team loses the fight, this enchantment lets to catch enemies that lingered too long.
Furor or Alacrity
, like Mobility Boots helps Fiora get to priority targets. Alacrity's initial MS increase helps her get within range to Lunge and are cheaper than Furor. Furot provide much more MS but are dependent on Fiora getting in range first. They both have their merits so which ever is fine.
Late Game Items
gives Fiora a slow to stick even harder to targets while providing health which is the best defensive stat in Season 3. Buy giants belt first as Phage is overpriced for its passive. Fiora's ultimate could potentially slow the entire enemy team with Frozen Mallet.
despite its nerfs is still a strong defensive option for Fiora. Fiora mostly relies on lifesteal for defense, so GA's resistances will make that health regained more effective. You can bait enemies by dying with GA, then using R to heal yourself but this is risky if they have hard cc or a snare. Get this against AD enemies as you'll need to hold on to a Chain Vest for a long time due to the combine cost.
takes QSS's great cleanse and combines it with more AD for an amazing MR carry item. It even has a melee only passive which provides MS so you can get away. Obviously great against mages with cc.
is omnipresent on tanks but its not a bad choice on carries either. Its on being hit MS and AS slow passive and its active makes Fiora a more potent duelist. Unlike tanks, Fiora is actually a priority target and will get auto attacked first. In a way, its basically Frozen Mallet with additional armor.