Skarner is a champ that has gotten a bad rap since he came out of being under-powered, unusable and/or just generally sucky, but with the right build and strategy he can easily become an integral part of most any team.

That being said, there are areas in which he can be very lack-luster so...on to pros/cons

Pros

Excellent chaser because of his slows and speed
General survivablitiy in lane
Completely infuriating tower hugger
Good team fighter (especially initiating)
Wonderful cool down reduction passive
Late game speed means easy full map control

Cons

Lack luster defenses mean you have to compensate
Stuns and the like can interrupt flow of abilities easily
Early game sustainability can be problematic
Need good map awareness to get the most out of this style
Taking on more than one enemy at a time may lead to tears (on your part)

Masteries
Both mastery builds work well on this build; it gives survivability which helps him be a good initiator in team fights and bolsters his mana regen which helps with the use of Shattered Earth

Runes

Three speed Quints to raise the catching power early game

For Marks it's all desolation for more early game physical damage

Seals in armor to make him more tanky and in mana regen cuz running out of mana is essentially your worst nightmare in this build

Glyphs are all magic resist

So I must say first and foremost that I'm just generally partial to the Ghost/Tele combo but here we go...

Ghost is there for escapes, ganking, map mobility and generally boosting your over all speed. I favor it over Flash mainly because the maintained speed increase that ghost gives works well in combination with both Seismic Bastion (turning you into a race car) and Impale (turning you into a long range grappling hook, pulling them toward turrets and your team).

Teleport is for maintaining early game turret resistance while getting new items to stay competitive in lane (especially if you get stuck soloing); late game it is used to turret hop to provide assistance and clear out creep build ups.

The gist of these summoner skills is mobility; this combo gives you short, mid and long range coverage.

Energize Energize- a great passive decreases your cool downs with each normal attack, because of this building attack speed is part of the general idea of this build

Crystal Slash- this is your bread-n-butter move, initially it does physical damage then on subsequent uses it does both physical and magic damage with an additional slowing effect. It has a two...ish... second cooldown which is lowered to almost nothing with his passive. Spam it well and often when chasing or farming or whenever you think its a good idea.
Crystalline Exoskeleton- Awesome ability use it for chasing and keeping yourself alive when your making a tactical retreat (fleeing like a little girl).
Fracture- Skill shot that does damage and puts a debuff on anyone in its path that heals Skarner when he hits them. This is leveled last because the amount of heath that you actually earn from it isn't super wonderful but taking it at 4 because it provides a consistent source of healing that allows long term laning
Impale- Freezes your target for a very short amount of time and allows you to drag them around. The dragging thing is nice for pulling an opponent away from a group or turret as well as pulling them towards yours, but the freeze is the real gem in this ability; it can render an opponent in a team fight completely useless for just long enough for the entire fight to be won.

Early Game (1-5)

Pick up Meki Pendant and 2 health potions before you head out. Play conservativly until you get 500 gold then go back and finish Chalice of Harmony. With that in hand running out of mana shouldn't be an issue so farm, catch the people in your lane if you can, but play smart till...

Mid Game (6-12ish)

Pick up Impale, shoes (either Boots or Mercury's Treads) and Zeal if your doing fine, Heart of Gold if your taking too much damage in general or Chain Mail if your taking too much physical damage...with that in hand start freezing ppl who over-extend, squishy casters, and anyone who has gotten too low and drag them to your turret or your lane partner or anything really as long as its away from any safe area for them while hitting them with Shattered Earth (It is important to note that keeping your slow buff up on your Shattered Earth is key). Chase with Seismic Bastion while spaming Shattered Earth and if they turn around to counter attack start hitting them with Ixtal's Impact just for additional health. Keep in mind that, until all the defensive stuff is built, it is an extremely bad idea to take on multiples of ppl or rush groups and the such so keep map awareness: know when to cut the chase, know when other enemies are coming to assist the person your trying to kill and know where your friends are so you can keep yourself alive if you need to run. So it goes till...

Late game (12ish-18)

Some speed synergy has been built up so start roaming (helping your allies, ganking, covering lanes, etc.). Remember to build Phantom Dancer it gives him that uber-deadly chasing ability. From here on out it's really just business as usual...kill...everything but below there are a few things to keep in mind, so...keep them in mind...

--In team fights try and freeze the problem character and dispatch them, generally a high damage dps or the like, BUT if its a tank that's being problem chances are he wont die before your freeze expires so instead drag them as far away from the fight as you can and start chasing them and using Shattered Earth, not to actually kill them but to slow them for long enough to keep them out of the fight
--Be wary of Gangplank his Remove Scurvy it'll beak Impale which will lead to tears (on your part) if you can get him to blow it on the slow from Shattered Earth then use Impale which will lead to tears (on his part) and probably a rude comment about your mother
--Watch out for super heavy dps's ( Warwick, Tryndamere); Never face them head to head their general life steal builds will just wear you down...

So...you may have noticed that with this build I only have five items; Allow me to explain, that sixth spot is generally in my opinion so situational that putting it down in stone on the build would actually be more regressive to the overall concept of this combat play style than helpful so I'm really just going to give some ideas and you can be smart and work from there...

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Guinsoo's Rageblade- pretty nice for spaming and not too too expensive

- Malady- pretty much the same idea as Guinsoo's but better if they are building magic resist

- Black Cleaver- if they are building lots of defense you can tear the down with this

- Bloodthirster- if your killing everything and just feel like doing more of the same but faster

-Another Phantom Dancer- same as above


All that said, I honestly prefer to leave it open for Vision Wards because, with the speed of this build, he becomes a very effective warder and general scout (who can also take care of himself) and now for...

SITUATIONAL ITEMS

The three things that i can think of off the bat are

-

Thornmail- if its all physical damage char

-sub Ward for Force of Nature or Quicksilver Sash if they're all heavy AP champs

-if you have a Fiddlesticks or other champs with fears and stuns and the such they can become impossible to chase so build Banshee's Veil to block those type of things

I honestly hope you enjoyed my exploration of this build. Seeing as this is my first Guide I would appreciate any comments and advice.

Happy hunting

11/9/11- Added alternate mastery and rune build and edited some of the text for inconsistencies

11/10/11- Adding special chapter to address situational items and that empty sixth item slot

1/3/12- updated the masteries and took off one of the builds cuz i didn't like it anymore