@Demonblader IMO, Last Whisper is tons better than The Black Cleaver. First, with TBC, you have to get a full THREE autoattacks on your opponent before fully utilizing the armor debuff. Now considering Riven has pretty terrible attack speed and animation, you probably would've caused less damage than if you had with Last Whisper and just going all out with your combo. Of course, you can say that you would get that armor debuff by just autoattacking in between a Broken Wings combo where each successful hit would make the next one hit harder. The damage would look something like this (please bear with my estimating skills): TBC - BW 100 -> AA 50 -> BW 110 -> AA 60 -> BW 115 -> AA 65, where BW = Broken Wings and AA = autoattack

As you would imagine with a LW, damage done will be much more reliable because the armor reduction applies directly to your autoattacks and skills, unlike TBC. With this, you would also become more of a well-rounded champion, being able to also take down most other tanks/offtanks/tanky dps. Also, in teamfights, riven does a lot of AoE damage. To fully utilize TBC in this situation would mean to autoattack everyone three times before unleashing a combo. With LW, you just blow your load, basically stay alive and finish any survivors with Wind Slash, never worrying about having to autoattack them three times to do the most damage possible. I hope that explains my choice and if you have more doubt, please do not hesitate to ask me more questions.

TL:DR
TBC, needs autoattacks to maximize potential, probably better in 1v1, wasted autoattak speed stat.
LW, better item overall, you are able to kill anything, better performance in teamfights.

@Blowfeld nice to hear! And don't worry, this guide will always have a place in my heart!

PS: Wow I wrote all of that on an iPod o_o

Hey, great to hear that my guide gave you some inspiration, that's like an up vote to my day...

Keep up the good work, a guide is never done ;)

Hmm overall nice guide, but explain why Last Whisper over Black Cleaver??
Because in my opinion you are not ANTI TANK champ. No vote yet will try play with Last Whisper.

Thank you! I've read through your guide and actually kinda sorta got inspiration to write a guide for Riven.

Regarding Last Whisper, I changed the item sequence a bit. I agree, Last Whisper should be held off until at least ~30 minutes (what with diminished percentage penetration with all the flat penetration already in this build).

Hi.
Your guide is easy to read and well formatted for a new guide. (if I would edit one more thing tonight in my guide I would start melting...)

I'd recommend Last Whisper maybe a little later and am no big fan of Sunfire Aegis but that's just personal choice ;)

+1 from me