Not Updated For Current Season
The masteries shown here are not yet updated for the current season, the guide author needs to set up the new masteries. As such, they will be different than the masteries you see in-game.
I am very aware that Ren is a very new champion and I have been playing him constantly trying to make the perfect build for this beefy damage dealer. I found one that works amazing. It allows you to keep carries in check always. I am not going to sit here and type a complete strat on how to use this hero. This is strictly for people who know how to use the hero but are lacking a good build.
Pros / Cons
Lots of survivability
Can run around easily through team fights stunning and slowing whom you please.
Not exactly a carry
Easily shut down by mandreds but you will not be the first target, so rip up the carry holding the mandreds while they target your squishies.
Later game build.
Ghost + exhaust is great for the intial chase to start smashing with the frozen mallet. Also great to toss on any carries with a mandreds.
Flash + exhaust is great for flashing into a slice ruthless and dice followed by cull then exhaust to seal their fate.
Ren is an amazing farmer. Pick any lane, because with SnD + Cull you can shred waves and waves of creeps.
Not exactly the most detailed descriptions but let me know you input and help me improve if you will.