Until you have your Hexcore upgrade on your E, You don't push backline minions in one ability, therefore, you don't want to get yourself in a situation where you have a very few seconds to clear the wave and not quite make it, but rather try to freeze the lane, using your Q shield to not get any damage from minions, deny enemy cs, and/or go for recall.

Since lost chapter is cheaper, you won't ever need tear for mana, and LC is a huge legendary item spike for Viktor which pretty much unlocks his ability to trade effeciently, and win even hard matchups through poke.

During lane phase you are trying to poke your opponent with E, as long as there is some objective to be reached, such as making the opponent recall before getting a certain item power spike, getting them on low hp so that they have to be afraid of jungler, poking them before taking major objective (tower/dragon...) etc.
However, your E mana cost is pretty high, so unless you have blue buff, you have to be very careful with how you use your mana.

Viktor is hyper-scaling carry, which means he is an insanely powerful late-game pick. Therefore, if you just get out of the lane phase even, you are always winning.
Enemy objective should be to either shut you down in early game or outroam you and win other lanes, until you to about level 11 / 2 items power spike.

If you play Viktor against smh like Qiyana, she might win the game until you get to your first item power spike, thats just something that happens in soloq once in a while, but you can try to avoid that by always pinging your opponent missing etc. And freezing cs away from her while she roams

With each Glorious Evolution(Viktor's passive) upgrade you get one basic ability improvement. At 3 upgrades, you have upgraded all 3 of your abilities and therefore your ultimates gets it's upgrade as well.

Mythics

Strong against tanky team comps, if enemy have 2+ tanks, buy it. You can buy this item later on in the game even if the enemy enemy has just one tank, it's value even against squishy champions is decent. This item also works well in extended fights thanks to it's passive that increases your overall damage the longer the fight lasts (up to 5s)

Strong mythic against squishy team, or if you are very ahead and want to snowball. Has better Mythic passive than Liandry but it's penetration can be compensated by MR builders quite easily.

VERY strong mythic against esp. assassins, and heavy meele comps that don't have long range abilities to proc this item.

Other items

This item is a must have if more than 2 enemy champions are building magic resist. You should build this item later on in the game no matter what since durability patch(as 3rd or 4th at latest) even if enemy players do not buy a lot of magic resist. The later on in the game it is, the higher value this item has.

Great choice against melees, also gives you movespeed which can make dodging skillshots and overall movement better. This item a style preference, as it alternates Viktor's playstyle a bit, but if you find success with it feel free to build it pretty much every game, just don't skip void staff over it if enemy is building MR and you should be good to go.

This item amplifies your damage like crazy, especially if enemy have shields OR don't build a bunch of mr.

Viktor is a heavily scaling mage, which means he wants to farm for about 3 items and then your heavy power spike comes into the play. Champions with such win conditions usually profit from Rabandon nicely.

You can go tear if you are playing against a tank in the lane, I'm not sure about tank mmrs, but in highelo you can experience pretty much anything, from malphite ornn sion to mundo in mid once in a while. In such scenarios, when the opposite laner is just eating your poke and hoping to run out of mana you can go tear for laning phase to slowly shoot him down. Howerver, don't put high priority on finishing the archangel's staff sooner than as 3rd item. Also keep in mind that this item is not very strong before it evolves into Seraph's Embrace, so don't finish it unless you have tier fully stacked.

Viktor has no escape as well as relatively slow cooldowns later on in the game. Therefore using your whole combo, zhonya into another spell rotation is very likely scenario in fights. But most important reason behind buying this item is Viktor's lack of escape.

Strong alternative to shadowflame, good especially if enemy are building mr.

Great item against champions such as Ahri Syndra or LeBlanc. You will lose a tiny bit of damage if you go this instead of other damage items, but the MR numbers it gives you are insane, especially against champions that profit from magic penetration, and makes you nearly an unkillable threat against LB especially. If you buy this item, you usually don't have to go Mercury boots and therefore you can opt-out for the ideal boots options - Sorcerer's Shoes. Also it Banshee's passive works AMAZING with Leblanc's kit

If your teammates don't want to build healing reduction and enemy has a lot of champions with healing in their kit (such as Dr. Mundo, Kayn, Olaf etc.) or many lifesteal items, you can build this. It's very strong item against champs who don't build any MR since it gives a nice portion of flat magic penetration.

Since the last rework on Viktor's abilities, you no longer need Rylai to slow opponents with your abilities, which is must have on Viktor. Howerver, if you are playing against a lot of champions that you can kite nicely (such as Singed, Kalista, Shen etc.) you can still technically go for this option, howerver is no longer stronger than other items you can get.

Boots

Default choice, useful especially if enemy don't build a lot of MR. Howerver, you can't always afford building these instead of safety boots. If you see early on that enemy team is already buliding a lot of MR, get some other boots instead and build Void Staff.

Good buy if you have 950 base, will always bring it's value. Especially good if enemy build MR (No Sorcerers angle) and you don't need Mercs either.

Safety choice you might end up going for most of the time. Must have if the enemy team has a lot of CC and you are not running cleanse. Great item especially against champions like Ahri LeBlanc or Syndra. If you don't need the tenacity, but just the MR part, you can just buy one null magic mantle into your inventory, build Sorcerer's Shoes instead and then finish Banshee's Veil later on. Finished Mercury Treads give the same amount of MR as just the flat null magic mantle.

Solid boots choice especially with team that you need to kite and/or have ton of slows but not much hard CC.

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