This build/guide is an overview of how I play Skarner. At first, I play him relatively squishy, and focus on lane survivability and damage output to get some kills and gain both a lane advantage and gold advantage. After finishing the initial item set, I go straight tank. Using this item build, during team fights I am almost always the first one in and the last one out, rarely dying, and dishing out some major sustained damage. Check it out!

Note: This is my first build, please rate it whatever you thought of it, I really do appreciate any feedback you guys have for me, and suggestions are twice as good!

Pros

Excellent early game laning
Is durable throughout the entire game
Has outstanding damage output mid game
An ultimate that both kicks a** and is bad-a**
Sustainable AOE slows and CC with exhaust and Impale
Very mentally threatening to other team after you drag them into your towers/team 10 times in 3 minutes thanks to your passive
Has excellent movement speed
Is incredible at focusing down one high value enemy even in the middle of a 5 v 5 team fight
Excellent farming capabilities
Is just flat-out fun to play!


Cons

Damage output early and late game is good, but not incredible
Has a period of "squishiness" mid game
His ultimate can be rendered ineffective by smart players
With the lack of AP in this build, his W and E abilities are not as effective (less powerful shield, close to non-existant healing)
Has a noticeable lack of Attack Speed without his W up (buff removed when shield is damaged off)
When taking defense items, you lose your edge for ganking champion during the later part of the game

My rune setup for this build is very straightforward. I focus almost entirely on early game survivability and dominance. I take the armor penetration and attack runes for both easier minion last hits and that little extra bit of damage for harassing. As I said before, I am focusing on lane survivability early on and so what could be better than a combination of armor and magic resistance? If you have played Skarner you have figured out that Attack Speed is a big part of his playstyle. The quintessences give that extra boost when chasing people with Crystal Slash / Crystalline Exoskeleton in early game.

MASTERIES NOTE::: I use a standard defense mastery setup with magic penetration for his spell spamming

Sorry for the wall of text here, this is the bulk of the guide, thanks to those that read it! Please don't vote on the build without reading it through! (If you don't want to read this, skip down to the summary which is much less in-depth and provides a solid overview of this section)
Start off with a Regrowth Pendant and Mana Potion. The Pendant lets you stay in the lane long enough to gather enough gold for your first trip back, which I will discuss in a moment. The Pendant makes it easier to get your next item, Philosopher's Stone, and the health regeneration is very noticeable in early game. Harassing eats through your mana like Cho'Gath is feasting it! After successful harass and maybe a kill or even two, that Mana Potion will give you enough mana to stay in the lane long enough for your next items. Try to save up 715 gold before going back your first time. (I have never had to go back with less than this in the many games I have played with Skarner) Go ahead and pick up your Philosopher's Stone (Great regen for the rest of early game and the beginning of mid game and the gold per 5 is just a nice bonus for the meantime) and grab your Boots of Speed. Head back into lane, or even jungle a little if you want as Skarner is an extremely viable jungler, but I simply prefer his lane game. Play fairly aggressively, and you should be getting your ultimate soon (if you don't already have it) which is AMAZING. His ultimate is godlike and it is his bread and butter for the entire game. Do NOT be afraid of Impale's long cooldown! With your passive, just slap a few minions and the cooldown goes from nearly 2 minutes, to as low as 20 seconds! People will RAGE at how often you drag them to their death! Grab a few kills in lane and once you have about 1300 gold go grab Sheen and a ward or two. This Skarner build is more of a support off-tank Skarner. (To build full tank, I recommend building the tanky items much sooner and skipping Trinity Force, which HEAVILY reduces your damage output) So in the spirit of supports, go ahead and grab a ward and place it at an area of focus, whether it be surrounding mid, or dragon, or your blue buff, or whatever it may be. Then, while your strolling through the jungle go ahead and gank a lane. Skarner's ganks are UNSTOPPABLE, because even if they are going to escape, Flash into them, Impale them, and Crystal Slash while you are carrying them, and then slash again as they start running to slow them. (They should be dead around now) You should notice a significant damage increase with Sheen. Your next item should be Berserker's Greaves (570g to finish). Now, save up, going back when needed and first build a zeal, then a phage, and then turn that into Trinity Force. Your damage will be pretty crazy right about now, however you are not fulfilling your tanky support role as of now. So its time to get on that. Save up 1500 gold and finish off your reverie, and then after that the choice is yours. Maybe the game is going great and you want to build more damage and just finish the game with your advantage. Maybe the game is slowly going downhill because your team is too squishy. Whatever the case, I listed some tank items above which I almost always use. (Sunfire Cape and Banshee's Veil) If you have these items and the game isn't over, your game must be crazily evenly matched. My games usually end as I'm either finishing Sunfire or starting Banshee's (or in reverse order). For the sixth item, it really depends on the enemy team, if they are AP oriented, grab a Force of Nature, if they are AD oriented, grab a Thornmail, if they are a great combination of the two, I recommend Guardian's Angel, or even just a Warmog's Armor to frustrate them with your insane amounts of health!

Regrowth Pendant + Mana Potion for survivability in early lane game
Head back with at least 725 gold and get philosophers stone and boots of speed for much better laning
Go back whenever you need to, getting berserkers greaves next (570g with boots of speed in inventory)
Then get Sheen for better damage(1335g for Sheen and Sight Ward, 1260g for just Sheen)
Go back whenever you need to for Zeal and Phage, then combine them into Trinity Force(In that order)
Next get Shurelya's Reverie.
(Tank Items Start Here) - Pick up Sunfire Cape and then Banshee's Veil whenever you can
6th item is open for you to get whatever you need most, I usually just get Warmog's if the game is really dragging out that long.

EDIT: I have tested in enough games to feel comfortable recommending Hextech Gunblade as a viable 6th item. This is recommended if you are tank enough to hold your own against their team, but need more damage.


Focus on laning, picking up kills in lane and from helping your team with ganks whenever possible. Good luck and have fun!