Don't buy it. You don't rely on auto attacks but on spells. The most useful Items are In this order:
- Your bulid (unchanged is, viable)
- Yomuus ghostblade (trade in Black cleaver/LW, can be viable)
- Trinity Force (Trade in LW, only viable if you're ahead)
- SotD (Trade in LW, may only be viable if you're way ahead)
I see some pros and cons with SotD on Riven:
- Stronger in 1v1
- Could be useful if you manage to kill some people fast in teamfights. (Kills/Assist reduce active CD by 50%)
- 100% Crit/AS which scale well with your ultimate
- The Item itself isn't really cost efficient. It's base stats are only worth around 70% of the gold and if you use the active and don't kill anyone you will even lose the passive AS
- Crit and Attackspeed are more or less wasted stats on Riven
- There are Items which are a lot better.
Now I feel like the points speaking against the Items are more important than the ones that suggest buying it.
Why shouldn't I buy it?
Let me clearly explain why you shouldn't buy it. Riven riven has different roles throughout the game. If you're playing top (what most Rivens do currently) you basically start the game as a Lanebully and duelist. You excel at denying farm and 1v1ing the enemy toplaner. However In mid-lategame you usually transform into an assassin and clean up champ. There your main job is to Focus squishy targets that are out of position or you join the teamfight rather late and you just clean up the remaining health of the enemy team. (If you're mid you will basically start as an assassin but the mid-late playstyle is pretty much the same).
Your main damage source is just plain attack damage. All of your skills have a rather good AD scaling. The scaling of your Q goes up as you max it and your ultimate has an execute that scales off with ad and it gives you 20% bonus AD for its full duration. Additionally your autoattacks have a 50% bonus AD scaling but they cannot crit. This is the main reason why crit is rather wasted on riven. Of course the attack that triggers the passive can but the bonus damage stays the same. So to do your job well pure AD is better since it also gives you more shield (which means more survivability).
What would be an alternative to SotD
Now I basically see two Items which are rather similar to SotD but I think only one would really be viable.
- Trinity Force
- Yomuus Ghostblade
Trinity force Is the Item I don't really consider viable but it's still a better choice than SotD. It provides you with a lot of burst (Sheen), Movementspeed (Phage, zeal) and AD + Health (Phage). However the downsides are that you will have 4 wasted stats and 2 of them are really compeltely useless on Riven. Crit and AS are rather wasted as I already mentioned while mana and AP are really 0-Benefit for her. The biggest plus really is the bonus mobility. I would only buy this Item if you're way ahead or you just wanna try something new.
Now this is a more viable choice. You can sometimes even see it in high elo on Riven. While it is still a situational Item I'd say this would be the best alternative to SotD. It has a very similar active with the most important pros of SotD but it also has cost efficent stats with AD, CDR and ArmorPen which are your most important ones. Its active will boost your mobilty aswell and not only your 1v1 and pushing potential.
What Item should I replace
The best Item to replace will usually be the Last whisper no matter which of the 3 Items you chose to buy. Your goal is to kill Squishies and they usually don't have 200 armor. If you go for a Ghostblade you can also trade in Black Cleaver for it but I'd still recommend removing LW. In the end the build you already mentioned is pretty decent and I think it would be the best to stick to it.