I'm new to riven and I can't really tell when to build lethality or when to build black cleaver/deaths dance.
I've seen both but don't really know if there's a reason to it (I assume there is though).
While we're on the topic of riven, what about thunderlords vs fervor?
Anything helps, thanks!
Broadly speaking, lethality is better versus squishies, and cleaver is better for those building armour. So if you see a tankless team, perhaps lethality is a better choice. (Occasionally people make the mistake of thinking lethality counters armour, which is backwards.)
Although cleaver/deaths dance is going to make you much more survivable, so you can be a bit scrappier instead of assassin-y. If you find yourself forced into a lot of extended fights with multiple damage sources, lethality riven might be able to burst one of the squishies, but she might have a harder time surviving to deal with the next one. Deaths dance/clever is going to make insta bursting a squishy harder, but you'll probably survive to try again.
That all said, I do not actually play riven.
Lethality if you're looking to be killing squishies for the most part.
DD/lifesteal if you wanna fight tankier targets (and LW).
BC is pretty much core though, even if you're itemizing into an assassin the stats and passive work against any laner you might go up against. Rushing Youmu(u)'s or Maw might be better in certain matchups though.
Fervor for long trades, Tlords for short trades. Fervor scales well against tanks and juggernauts, while Tlords is more effective for wombo's and assassinations.
