Relics: Blink, Thorns
Starter: Pick Watchers Gift/ Mark of the Vanguard depending on what you play and also your personal playstyle. I'd recommend Watchers for supports and Vanguard for Solo lane (or other modes like Joust or Siege).
1st Relic: Blink is for Blinkpulls or to get into the opponents without having to waste your 3.
2nd Relic: Thorns for obvious reasons. You want to reflect as much damage as possible.
Shoes: Travelers Shoes is the first choice here since you really want to hoard gold to work on the rest of the build. Feel free to use Shoes of Focus instead if you want some early CDR. Sell Travelers for Focus when you finished your build (or reinforced shoes if you take the 40% CDR build).
Mystical Mail is likely the better choice if you're playing Solo since it helps alot for waveclear. Breastplate of Valor is a cheaper valid choice aswell. If you can't afford Mystical or struggle with your mana take BoV instead. BoV might also be the better choice if you're playing Support so you don't steal lasthits passively or if you don't want to risk drawing aggro from objectives. Shorter Cooldowns help to reflect damage more often (and reducing the cooldown on your NeneKappa(1) a huge amount since you still get that sweet flat CDR on it by reflecting damage with your 2). Reflecting damage more often also helps in dealing more damage. You're also more supportive to your team if you have your CC more often compared to that damage aura of Mystical Mail. I personally feel like the amount of dmg done is pretty similar between those 2 choices. Mystical is mainly required for waveclear and a bit of dmg when you're out of mana.
Void Stone gives you magical defense and helps to boost your damage output by lowering the opponents defense (works well with Mystical but is still an amazing pick if you take BoV instead).
Nemean obviously boosts your reflection damage even more.
To get more magical defense, take Pestilence if you need antiheal, otherwise Genjis Guard is a good choice aswell, for previously mentioned reasons regarding CDR. If you took BoV and Genjis feel free to spam your abilities as much as you want. Your cooldowns are short and your mana rec is huge.
Last item is situational. I personally enjoy Mantle of Discord for the CDR, mixed protections and stun/CC immunity so you're able to escape if you ever get low on health. Stone of Gaia works well too if you prefer life rec. If your team gets a good amount of kills Mail of Renewal is amazing aswell. Hide of Urchin still works since it gives protection and health but i wouldn't recommend it since you'd need to stack it as the last item and you're better off with Mantle of Discord at that point.
Don't forget to replace your Travelers Shoes for Shoe of Focus (or Reinforced Shoes if you already capped CDR). You don't need power on Kuzenbo. Your 1 has a good amount of base damage (and the Kappa basic attacks don't even scale with Power at all) and for reflection damage you only need CDR so you can reflect as often as possible.
Build 3 is an alternative build that sacrifices some CDR for relic dagger. Relic Dagger reduces the cooldown on Thorns from 120s to 90s which can be useful aswell. It also offers ton of HP.
You can replace Winged Blade for Shoes of Focus if you don't feel the need for its passive or the HP and want some CDR instead.
Get Pestilence instead of Void Stone if:
a) You need antiheal.
b) You're behind and need to get as much defense/health as possible. Pest has 10 more magical def and 50 more HP than Void Stone for the same price.
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