With this build, I will show you what I think is the best build that accents Renekton's free spells. I will be focusing on a Damage and Cooldown Reduction based build and strategy. This style of Renekton focuses on using his spells and abilities to damage his opponents. The best way to do that is to catch up to an enemy champion and burst him with powerful spells. This can lead to a massive chain because of his Cooldown Reduction from many sources, and can't be counters by a single item like Thornmail. This Build is supposed to be DPS mostly but it can be used as an off tank build by changing some things. I hope with this guide you can strike fear into the enemies and win.

Since this Renekton build focuses on bursting the enemy champions with increased Attack Damage, I would recommend a build similar to this one.

Greater Mark of Desolation

x9

x9

Greater Seal of Vitality

x9

x1

x1

Greater Quintessence of Desolation

x1

These runes give a focus on bursting, dealing damage with your spells, and actually penetrating their defenses. The Greater Quintessence of Swiftness is there to make it easier to catch up to someone but can be replaced by a .
An off tank build may consider something more along these lines.

Greater Mark of Desolation

x9

x9

Greater Seal of Vitality

x9

x1

x2

The second style has a small change in Quintessences for more health.

These are fairly important and also give more power to the focus of this build. I have a 21/9/0 Mastery build focussing mostly on Attack and dipping into Defense. In my Attack Masteries, I grab Attack Damage and Cooldown where I can and grab increased damage to minions so it's easier to last hit and farm. My Defense Masteries consist mostly of Magic resist and Armor with some Health Regeneration to help Renekton's survivability increase.
An off tank build my wish to add more points into Defense but this Mastery build should also work on off tanks as well.

These are the spells you will be using as Renekton. These will either make you or break you. This is a little bit flexible because of how you play but some spells just don't really work.

Good Spells

This works very well for chasing or escaping as Renekton's only escape is through runes or his spell Slice and Dice.
Exhaust is great for anyone trying to stay alive or to weaken their prey.

Spells to consider

Flash is a personal favorite of mine. This helps a lot if Slice and Dice is on cooldown or if Slice can't go through a wall, but really it's just another Slice.
Teleport is helpful if you need to get somewhere very quickly because Renekton is fairly slow and clunky.
Cleanse is nice if you get ganked a lot. It also saves your hide if the enemy team focuses you with their CC because they're too scared of the Croc.

Other spells aren't quite as useful but the choice is mostly up to you.

Rundown of Buy Order

At the start of the game you are going to want a Boots and 3 Health Potion.
This is great because Renekton has nice beginning gameplay and is hard to kill only if he can get at the minion wave. If you are being attacked, the Boots will save you from most hits and the Health Potion will save what does get hit. Last hit and use Cull the Meek on the minions until you have 700-2037 gold.
Your next buy(s) should be The Brutalizer and Ionian Boots of Lucidity. This early item build is great for Renekton. He'll have enough speed and cooldown to stay alive, while having enough Attack Damage, Cooldown, and Armor Penetration to actually burst down a few kills. After last-hitting minions and maybe pushing a lane for awhile, you can go back when you're low or swimming in cash and buy Spirit Visage and B.F. Sword.If you bought the Spirit Visage and don't have quite enough for the B.F. Sword, you can go ahead and buy a Vampiric Scepter.
The Spirit Visage let's you stay alive longer while the cooldown theme reoccurs. The B.F. Sword lets you really start hurting the enemy while a Vampiric Scepter can help you burst back up to full health. Either one is viable. At this time you should be somewhere in the Mid Game so lots of fights will erupt. When you have enough money you can go back and get one of these 3 items.
Bloodthirster
Black Cleaver
Frozen Mallet
Bloodthirster can help dish out some damage while keeping you well fed with life steal. Black Cleaver is a threat to any one getting hit by Ruthless Predator, as Renekton can then hurt them even more, even tanks. Frozen Mallet lets Renekton chase more easily as well as keep almost anyone in range after a Ruthless Predator. If you really want to or have the extra money you can go ahead and upgrade The Brutalizer into a Youmuu's Ghostblade.

Plausible Items

Banshee's Veil would be a nice alternative item if the enemy team has a lot of CC or AP.
Sunfire Aegis is a nice alternative item to Frozen Mallet as it is like a second Dominus.
Guardian Angel is a nice alternative because it raises your overall survivability and you get a second life.
Hexdrinker would be nice alternate to Spirit Visage because it gives damage and makes mages second guess bursting you down.
Madred's Bloodrazer is nice because it adds magic damage to everything you use so they can't just use Armor and beat you.

Dominus> Cull the Meek> Ruthless Predator> Slice and Dice
Dominus is a great tool and is the ultimate so level Dominus when you can.
Cull the Meek is your best tool early game so you don't die. Later it gets great damage output along with a Fury and Health input. Level Cull the Meek first.
Ruthless Predator is a fantastic skill that not only severely wounds the enemy but stuns them in place. This applies on-hit effects like Frozen Mallet and Madred's Bloodrazer. Level Ruthless Predator second.
Slice and Dice is a great tool for harrassing, initiating, chasing, running, and damage. level Slice and Dice third. By level3 you should have 1 level in each of Renekton's abilities. His diversity will be great with those three. Your Overall sequence should run something like this.

Cull the Meek Ruthless Predator Slice and Dice Cull the Meek Ruthless Predator Dominus>>> Slice and Dice Cull the Meek Ruthless Predator Cull the Meek Dominus>>> Ruthless Predator Slice and Dice Cull the Meek Ruthless Predator Dominus>>> Slice and Dice Slice and Dice

(I have punctuated the sequence with >>> every time your ultimate is leveled for more organization.) I like to broaden Renekton's horizons by having the spells run pretty even until level 10.Then I usually focus on Cull the Meek and RuthlessPredator with a few more in Slice and Dice. This way they are all fairly powerful and useful in each of their uses.

Early game sees Renekton as a good farmer who's only goal besides money is staying alive. Use Cull the Meek frequently in this stage as it gives you health and is a great farming tool. Most of this stage involves farming and staying alive with Cull the Meek, as well as harassing enemies with Slice and Dice and Ruthless Predator. Push the lane you're in, most likely being top or bottom, and try to get a few kills. Don't be so aggressive as to become ganked as soon as the Mid Game starts, but harass enough as to try and zone the enemy or keep them tower hugging. To sum up the Early Game, just stay alive, get money, and do a little bit of harassing.

Mid Game sees the entrance of ganks to our list of things to do. Check. You can either gank mid lane or the far lane with a mean combo of Slice, Ruthless Predator, Cull the Meek, and Dice. Hopefully you gave your teammates a heads up so at that time they can rush in and help get the kill. If you yourself are ganked things change a bit. If they are really close you can use Ruthless Predator on their greatest weapon against you. That, for you, is silence. If they caught you off guard but they are in no immediate threat, you can turn tail and run towards a close thin wall, thin being the key term, because you can Slice and Dice through it to safety. If any large fights occur, like they usually do, you can use the same combo as if ganking,except that you use Dominus and then Slice, Ruthless Predator, Cull the Meek, and then Dice out. Make sure to target their best weapon when using your Ruthless Predator. Everything you'll be using will either give you Fury or use it up so burst like crazy on either a Mage, a DPS, or a support. Smart target kills will win you the fight.

At this point you can probably handle any minion horde on your own with Slice and Dice and Cull the Meeks. Epic monsters and enemy towers are big targets at this point and if you can do some damage to an Inhibitor or it's turret and Slice out, that would be great. Team fights will need you to stun their threat, and do as much damage as possible before hopefully killing them all, or unfortunately, dying yourself. If your team is going to have a huge fight to the death, it means don't run away. Renekton can do more damage fighting than running, leave the retreat damage to Teemo, Singed, Anivia, or someone with a skillshot. Just make sure to fight smartly and help your team until the end as you should be a major source of damage.

Renekton, the Furious Butcher is a guide based on his skills so I'll go through a rundown.
Cull the Meek is usually used to gain health early on but later used as a damage tool, either while chasing or just for the Area OF Effect damage it deals.
Ruthless Predator Is usually used to stun any threats and usually retains that use, and later is used to use on-hit effects as well as large amounts of damage to single target.
Slice and Dice is used for many things; Chasing an enemy to close the distance, running away through the jungle by slicing through small walls, damaging minion waves or a target champion, lowering a target's armor if it has rage, dodging skill shots, initiating fights, or harrassing an enemy (with a Slice and Dice in with Slice, use Cull the Meek and/or Ruthless Predator, and Slice and Dice out with Dice within the time frame.) Remember that using Slice and using an unneeded Dice will have a longer cooldown than just Slice.
Dominus can either be used to tank a turret or team, use the AOE damage in a fight or minion wave, or use it for the extra Fury it generates. Just don't waste on what you initiate if it won't even be a fight. They may just run or tower hug.