Like in many guides here i want to say first: This is my first guide ever. Also english is not my native language (it's german actually as i am from austria) so my grammar and spelling may or may not be perfect i dont care at this point.

I played renekton quite much since he arrived to the league and tested him many ways. Offensive/Defensive and mixes with focus on different things (like more offensive items with defensive masteries or the other way round)
At the moment (as i expect some changes next week... hoping for a buff buts thats another discussion) i think you are the best help for your team as and offtank. Sure he is not bad and fun to play offensive and i also had some epic games with this playstyle but at the end yor team will be happy about and offtank renekton.

Abbreviated Terms

AoE = Area of Effect
CC = Crowd Control (Stun, fear, slow, snare, etc)
CDR = Cooldown Reduction
ArP= Armor Penetration
HP = Health
MR = Magic Resistance
DMG= Damage

Pros:
- Great Farmer. just run into minions waves use Cull of the Meek followed by slice and Dice and enjoy the ringing of gold
- Stun with Great Dmg and not so high Cooldown
- You can take some Dmg and Deal quite a bit too. Slice through tank and melees and eat/stun the squishys omnomnom
- Nice Aoe dmg with Cull of the Meek and your ultimate Dominus
- Nice escape and chase with Slice and Dice. Double Dash! Its so beautiful! What does it mean?
- Nice harass During laning Phase
- One of the most stylisch champions especially with galactic skin

Cons:
- You may have some problems early game. getting kills wont be that easy As your CDs are quite high on low ranks and you dont have a good finishing blow
- There are better dmgdealers
- You cant go full tank

Marks: Armor Penetration: Any Questions? I didnt think so
Seals: You can chose among: HP/lvl (Good late game), Dodge (good eraly game and with def mastery), and armor (also viable especially with MR Glyphs) but i would chose HP or Dodge)
Glyphs: Cooldown Reduction( so good with renekton), if you think you can get enough with items (its 40% max dont forget) go ahead and take MR. If you think you want more CD take the per lvl ones but i think its really useful early game.
Quints: Flat Health. Not that squishy eraly game. Maybe you could Change one for more ArP

8/21/1

Off: More CDR yey! + improved Exhaust and some Crit. If you dont use ghost go ahed and put 1 point in Sunder. More arP is always Nice
Deff: If you Chose to take Cleanse take the improved one for 1 point from Ardor
Utillity: improved ghost because why not if you take it.

You could probably also go 21/9/0

i wont explain the skills to detailed now ill make it later on

Cull of the Meek:
Use it to gain Fury, 25 fury from hitting 5 minions or Champs is nice and you will be able to use 50Fury Ruthless faster.
Use it with Fury for farming or if you need health. otherwise you want to spend ur fury on

Ruthless Predator:
This is the skil guys. Nice dmg and stun and if you have The Black Cleaver you will have 3 Stacks instantly on your target. Try to ALWAYS use this with fury. if it saves your or your teammates *** if you use it eralier do it but in teamfights you want to spend your fury on this

Slice and Dice
Not so great Dmg but good for chasing/escaping/dodging and Dashing through thin walls. Use it in teamfights to Slice your way to the squhisy to use Ruthless.
You unfortunatly Cant Dash through the walls of your or your enemys base.

Dominus:
Your Ultimate. Deaks AoE Dmg and works great with sunfire Cape. Also you gain fury each second, get some life and you will be HUGER THAN A FING XBOX!
You are not invincible with this. I often make the mistake to do crazy stuff with ult on.
Pop it when u are sure there will be a fight and use it after you go in. if the enemys see a huge Crocodile God running towards them they will maybe run and you have wasted your ult and stand on the lane feeling dumb or they think "omg focus him down!". you want to take some dmg but not all of it.

The items above are just an example. After you bought brutalizer youll have to think (oh yeah) which item would be best for you. if casters cast you down maybe get a hexdrinker(more Offensive) or a Force of Nature(if they really cast you down friggin much). If its a mix leaning towards physikal dmg (auto attackers) get Randuins Omen or if you wan to be more agressive Sunfire cape(more for when you are having a good time).
if you dominate and dont have problems with dying feel free to take The Black Cleaver or at least a B.F. Sword earlier.
Youll have to think what your team needs and whats bad for you oponents.
Thats so improtant for offtank renekton.

Other great items are
guardian angel
Warmogs Armor (Combine it With Atmas impaler)
Thornmail
Quiksilver Slash
bloodthirster (you can take it instead of The Black Cleaver or throw your Brutalizer away for it)

The thing with Youmuus Ghostblade:
I really like the brutalizer on Renekton. Gives him all he wants for beeing offensive and isnt that expensive
But im asking myself if its worth building it into Ghostblade. Some more CDR arP and AD is nice but is it worth the money? you Dont need Crit that much and its active...kind of meh.
I just think there is a better investment for your money like sell Brutalizer for Bloodthirster.
if i get it i get it last because you dont need it early on(or need your money for other things)

Dont initiate as you will get focused down very quickly. Go in after the tank/ after some CC and pop Dominus. Slice and Dice your way to the sqishys (as they usually stay in the back and Stun the most dangerous one. Hit them, eat them, make then run around aimlessly in fear. yo can move through a Fight with slice and dice thats you bid advantage and makes you a great use for the team.

I will improve this Guide as i find out how you make symbols usw and as i have more time xD