This is my first Guide and I don't say It's the best guide for everybody and I don't say You will always win with it, but I usually do. I would recommend it to beginers and slightly intermediate, because that's about - normal plays and low rankeds, where people usually don't go all tanky, but if they all go, they won't do (much) dmg to you with this build and you will heal against them.

The reason I made this build is unkown to me, because I think it will make a lot of rivens a lot better and it will be harder for me, to play against them. :D

This build is made out of 40% cdr at lvl 18, lot of life steal, and some tankiness. This build is well balanced, but if you feel there are better ways to play riven (and I don't say there are not,) you can play it, but I feel this build is safe and it is winning.

Some explenations:
First thing is why I didn't put the last whisper in, because it is so hard to change it for anything in this build, because you will surely lacks on something. I think everything in this build is just so balanced. Mainly you have a some HP, some armour and some magic resistance. You have all the cdr you can have and you also have the life steal you need and it's even increased with 20% of spirit visage. And you also have 250% base health regeneration, which is true that it's not really necessary on riven, mainly because she has almost no base health regen, but sometimes with potion it can make you some kills or it can save your life.
Guardian angel which doesn't have any damage, or cdr, or life steal it still speaks for itself, it gives some armour and magic resist and it will revive you and after it your cd's will reset so you can stun (with hydra which will also heal you for a lot) your oponents or use your windslash, which is sometimes so important in teamfights.

I just thought I will say something about them, because I didn't write anything in desription.

Black cleaver - must have item - it gives you 20% CDR. ALSO it gives 400 life, which is not bad at all. 30% armour reduction(so OP)!!! and it gives you movement speed - just why NOT?!?

Ravenous hydra - 12 life steal - Which is pretty good and gives you nice sustain. THE CLEAVE! - it gives DMG to all your enemies within even more than your reach and it also can be actived! (WHICH gives 100% of you attack damage!) - THE DAMAGE!! - if you activate it everytime when you use your w, it gives horrible amounts of dmg... (and also the 100% health regen - not bad early.)

The bloodthirster - THE DAMAGE, THE LIFE STEAL and THE SHIELD - what there more to say

Spirit Visage - The thing is that this item just IS completely PERFECT for this build, because it has the last 10% cdr you need and it also gives you 20% more of your lifesteal and it also gives you 150% of your base health regen - which I think is not that important - mainly when you usually build this item in late game. You will also get 400hp and 60 magic resist - good to have.

Guardian angel - usually last bought- it resets your life, it gives you both 50 armour and magic resist. - Just seems fair.

And the thornmail - can be replaced for guardian angel - if the team against you have somebody as - yi, vayne, aatrox - it could be better than guardian Angel, but guardian angel is kinda fun.

Banshee's veil/spirit Visage - Banshee's veil has little bit more health, same magic resist and has an interesting passive(mainly against AP champs), but it doesn't have CDR!!! and it doesn't have 20% more lifesteal

Brutalizer can be good sometimes against some oponents, but you can't upgrade it to anything else, but
youmuu's - which is not good for this build, but if you will sell the brutalizer after, you can do it, but you will loose some money.
Ok - why to buy B.F.Sword and why not to buy it. - It has no sustain at all, but you will do masive dmg so if you are ahead and you feel confident go for it, but it's really squishy and you will have to buy some potions with it at least - pickaxe the same but if you had to back sooner for some reason.
Kindlegem kinda lacks of any dmg, but 10% cdr - will give you some dmg, because you will have your abilities faster, which is so IMPORTANT - (stun, q, shield, PASSIVE - faster ult)!
Now the vampiric scepter - I know it has no CDR, but it gives you some life steal and some dmg, which is pretty nice - and it makes you stay longer in the lane.
Long sword and ruby crystal - it's those things if you don't have enough money to buy anything else and you had to go back or you died or something.