I AM NOT CLAIMING, THAT THIS IS THE BEST BUILD FOR EVELYNN OUT THERE.
I AM JUST PROVIDING YOU WITH A NEW FUN WAY OF PLAYING THE CHAMPION, SINCE
LETHALITY IS THE THING NOW AND I TRIED PLAYING ALL SORTS OF **** WITH LETHALITY
SINCE IT EXISTS.

THANK YOU FOR READING THIS!

With the introduction of the new assassin items, we got a new stat called "Lethality".
Even though it sounds quite fancy is basically a glorified version of armour penetration, what makes it unique though is the interaction with other heroes and a certain item, Duskblade of Draktharr. This item allows us to get 200% worth of lethality as additional true damage and based on how lethality works, it basically gives us additional true damage for building ad + flat true damage.

There might a question you all ask yourself now:
WHY EVELYNN!?
And I want to answer you with a question myself:
Why not?

We are basically scaling the damage on our Ravage (E) into a disgusting nuke, that deals a huge part of its damage as true damage. And that being not enough, we also have the ability to mindgame the enemy, since nobody really expects an Evelynn to build Lethality.

Pros:

Fun Way To Play Eveylnn
Insane Single Target DPS
Possibility Of Being Bulky Assassin
Lethality Swag
Insane Movement Speed and Map Presence (For An Evelynn)
Ability To Break Out Of Slows
Invisiblity


Cons:

Can Have A Hard Time With CC Tanks
Easily Locked Down By CC
No Direct Gap Closer
No CC
Important Early Game

Runes and Masteries are pretty much self-explanatory.
For Marks we take Lethality, just as we do for Quints, since it is one of the few ways
of scaling Lethality in the game.
For Seals and Glyphs we go for the best defensive option possible, being flat Armour Seals for early survivability, while jungling and flat + scaling or pure scaling
Magic Resist Glyphs, since we fall short on the defensive side in terms of items.

Regarding the Masteries, we are going 12/18/0 to grab some good nodes for Evelynn, which are Precision, Natural Talent and Thunderlord's Decree, Double Edged Sword. Precision and Natural Talent basically allow us to scale both of our damage sources, with Precision even scaling up our Lethality. Double Edged Sword is an important node, since we rely on our early game and the boost in damage early really helps. Reasoning behind this is, that we want our damage as early as possible so we can't rely on getting 3+ kills on different champions to scale Bounty Hunter, neither do we have long enough fights to make Battle Trance worth it.

GANKING
In terms of ganking the build doesn't really differ that much compared to your textbook Evelynn. You sneak up invisible via lane/brush/river and just collapse on the enemy once they've crossed a certain line. You only use your Agony's Embrace (R), if the situation really asks for it or the enemy has some highly mobile chmapion, that gets locked down by the slow it provides, since it isn't your top priority damaging ability and doesn't scale up in damage, due to the build.

Mid/Late-Game Fighting
The fighting in the later stages of the game is what makes the build really fun to play. Since you don't really as much on your Agony's Embrace (R) to take a huge chunk of the HP of your enemies and you have a insanely mobile champion once you go stealth.
The whole map is basically your playground, where you prey upon unsuspecting targets.

Due to the unique interaction between Duskblade and vision-breaking it allows for a hit and stealth playstyle. Try breaking vision as often as you can with your enemies to reapply the passive effect of Duskblade as much as you can. You have an easy time catching up to enemies, if they try to run due to your massive amount of movement speed once you leave combat and even in combat, you are hard to escape from, because you have your Agony's Embrace (R) and your Dark Frenzy (W) to chase down enemies. Talking about Agony's Embrace (R). You don't want to use it to engage fights, unless you can get a big ultimate on multiple targets, because you more often want to use it to hinder running enemies or get an additional shield in longer fights, because of our lack in defense.

As for dealing damage, you just spam the **** out of your Hate Spike (Q) and try to proc Nightstalker with Ravage (E), because it is a reliable way of getting out the most of your damage. Don't ever forget to Auto-Attack, we scale a good amount of AD and Lethality, so your hits hurt!