Build one if when you have at least 2 tanks in your team. For example bot lane Alistar or Blitzcrank as a support and Malphite or Volibear top lane. You will be doing damage as much as your carry does.

Build two is when you have only one tankier champion in your team or when you just focused a lot more. You will be tankier, while still capable of doing insane damage.

Pros:
* Amazing jungler
* Amazing ganks ( can gank at lvl 1 )
* Hard to notice without vision wards. Even if they place a vision ward they won't have much more gold for items.
* Very fun to play

Cons:
* Not very fast jungler
* Squishy
* Often focused if doing well
* Most of the people think she sucks

Other viable summoner spells:

It gives Evelynn some extra toughness in the jungle, and you really need it, because she is very squishy early in the game. Take this and later upgrade it to Madred's Razors.

This boots give Evelynn that roaming power - they make her fast so she can go anywhere on the map, whenever it is needed.

Wriggle's lantern helps with clearing the jungle faster and you will kill Dragon and Baron faster. Plus the free ward is also very helpful.

Evelynn benefits from every single stat this item give you. Start it of with the Sheen, then take your Zeal and last Phage.

Some extra AD, armor, attack speed and a passive who will melt tanks away. This is one of the best passives ingame because just think about it. An avarage tank will have around 3000+ hp and 4% of 3k is 120. And thats a lot for just a single attack. With the AS boost from your items, you will attack quite fast. That way no enemy can stop you.

The best AD item ingame. It gives you AD, critical strike and your crits will hurt more. Just an overall awesome item.

Item which is a must for Evelynn. It gives you movement speed, atack speed and crit. She benefits from everything. I get it late in the game, because it for final touch to an unstoppable killing machine.

Shadow walk (passive):
Evelynn stealths into the shadows only being seen by nearby enemy champions or true sight. While stealthed, Evelynn regenerates 1% of her maximum mana every second.
Casting spells, taking damage or dealing damage reveals Evelynn for 6 seconds.

Hate spike :
Evelynn fires a line of spikes through a nearby enemy, dealing magic damage to all enemy units in its path.

The main spell of Evelynn. This skill scales good, both with AD and AP. I max this first for the good damage output on low cooldown and it helps a lot in your jungling.

Dark frenzy :

(Passive): Evelynn's spell hits on enemy champions grant her bonus movement speed for 3 seconds. This effect stacks up to 4 times.
(Active): Evelynn removes all slows affecting her and gains bonus movement speed for 3 seconds. Champion kills and assists refresh Dark Frenzy's cooldown.

Your speed boost. This combined with your Mobility Boots will result 500+ movement speed. And the best think about this skill is that its cooldown is refreshed on kill or assist. Go in teamfight get a kill or an assist and go away with the speed of the wind.

Ravage : When used, Evelynn will atack twice. Simple as that, yet so helpful. The best thing about this skill is that when it is used u get bonus AS. This is helpful when pushing a turret - use this skill on a minion and then attack the turret with amazing AS.

Agony's Embrace : Useful for the slow and the shield. With the slow you can escape bad situations. In teamfight use this and you get instant 1500 shield at max rank. Just try to land it so it can hit every enemy.