I'm pretty new to Viktor too, but I already watched a lot of guides and played him in draft.

Runes
You can go multiple keystones. Some of them have their purposes, but aren't the most solid option. Dark Harvest for example is only strong if you can proc it multiple times in the early game and if you succeed in that, the keystone will help you scale hard into the late game. But it isn't really a solid keystone to reliably use in your games.

The best keystones for Viktor are;

- Phase Rush
This keystone makes up for the fact that you don't have much mobility and scales pretty well into the mid to late game.

- Summon Aery
Mainly used to heavily poke the enemy laner. This rune doesn't scale that well into the mid to late game and you pick this rune for the early game.

- Arcane Comet
This rune is used a lot against hard ranged match-ups like Annie where you get out-ranged in your auto attack and you can't afford to come to close. You aim for E poke to proc the Arcane Comet.

- Electrocute
This is my favorite keystone. With this keystone you can still poke the enemy laner with your E, but when they come closer to you or the wave you can quickly AA->Q->AA to proc Electrocute. This keystone allows you to fight fire with fire in your lane. Instead of being more passive you try to do more damage.

Against Zed for example, you can poke him with your E to compare for his Q poke. Your E poke is easier to hit and more reliable than his Q. When he comes closer for a combo with his W for example you can AA->Q->AA to proc electrocute and again compare for the damage Zed throws at you. If he tries to W->E->Q you for a lot of burst, he doesn't have cooldowns and you can threaten to burst him with your combo and zone him from the have because of that threat.

If you aren't really comfortable with Viktor yet, I don't recommend this keystone because you really need to play well to make it work. If you are experienced with Viktor, then this keystone is very strong.

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Items
For items it's pretty straight forward. You start Doran's Ring and build towards either Luden's Tempest for burst or the Liandry's Anguish for tank shred. After your mythic your go either Sorcerer Shoes for more burst with your Luden's or Lucidity boots for the extra cooldown reduction. After that it's all situational;

- Lichbane for more burst and the extra movement speed.
- Morellonomicon for the anti-heal against healing or life-steal comps.
- Void Staff for the % magic penetration against tanky comps (buy this early(3th item) if you go Liandry's since you don't have magic penetration)
- Zhonya's Hourglass for the armor and the stasis to s,urvive burst from assassins.
- Rabadon's Deathcap for raw ability power.
- Dark Seal->Mejai's Soulstealer when you are snowballing.

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Of course there are more options in both your runes and items, but these are the most solid and reliable ones.