Renekton, Butcher of the Sand and Cooldown death machine. Ren has an Amazing skill set, a stun to dash to catch, stun, to allow others to catch and lastly a great lifesteal/dmg swing to help in those tough situations. And don't let me forget that ult he shares with his brother practically which buffs the swing to annihilate in a team fight and give almost all hp back.
Now this build is basically a skill users quick fingers at work, i'll explain further later.

Now some runes are just a matter of opinion.

Marks: This is my matter of opinion rune, The dmg is nice but there are plenty of great alternatives, you can go armor/resist or anything you want i just choose dmg because its a great starter and helps in the early game with some of your skills while your building.

Seals: Pricey yes but seals of fortitude offer a great advantage since you miss out on the defense mastery lane, they allow you to not only either stay at a reasonable level of health against tanks but blow out the water your enemies that are lower than you.

Glyphs: I Believe these are a necessity wether you decide on per level runes or flat runes i suggest CD reduction, like i stated before Ren is a skill based champ with a good dps factor in there too allowing him to be able to skill and do insane amounts of dmg output while still healing himself and allowing a nice get away.

Quints: They follow the same as Glyphs.

8/3/19
Sounds a little odd but just try it out.
In the offense mastery lane i pick up crit chance just because ap isnt very used aside from Ren's ult and plus he is a Dps/skill Tank. Including the CD reduction, and no longer progress after that seeing how for some reason not only does ignite mastery not work it's useless and so is the magic penetration.
Defense Mastery lane it's up to you, you may either go Armor or MR, completely up to you, or not at all if you would like the atttack speed in offense mastery lane.
Lastly Magic Mastery Lane
I push up all the masteries that don't solely deal with mana, grabbing buff time increase, movement speed and ghost which are both very handy when trying to grab that runner. Lastly i end the masteries with the CD reduction again, giving us a total of 9% just form masteries.

At game start grab a ruby crystal, which starts you out at around 760 health. As you lane try to as always stay careful and call MIA's :). after laning for a good while and getting maybe a kill or ace on the bottom go back to base and grab the nice new boots granting you 15% CD Reduc.
As time goes by around level 6-7 you should be able to go back once again and get a phage or brutalizer your choice but start on the other if possible. Not oly does the phage give you some decent health that slow combines with your stun and allows for an awesome quick kill.
after obtaining both dependant on how your being harrassed grab a spirit visage or a giant belt (for frozen mallet later). Now if your doing ok with everything try for ghost blade, it has an amazing activation, good damage and combines with every skill and item you get later.
After getting all the items shown it is solely up to you to decide what you want next, Black cleavr is very good and works with W well, Bloodthirster goes with your minion reaping Q amazingly allowing a quick rise in dmg output and lifesteal. Other than those you could also go a tanky style and grab warmogs and FoN.

I really hope this is a good build for many as it has been for me so far, now wether a buff is incoming lets hope not but have fun playing the game and don;t knock it til you try it :)
Lets look at CD reduction fom everything
9% masteries
15% boots
15% from ghost blade
12% from spirit visage
11% from runes
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62% cd reduction :) have fun