Riot said Lethality items/builds were a bit weak, and ends up nerfing the items and the synergies between items overall. Youmuu's was slightly buffed, Duskblade was significantly nerfed (particularly the passive), and Edge of Night got a small nerf. Overall, Assassin build paths are also a lot weaker.
It was previously stated by Riot that lethality items are a bit weak right now, and I'm assuming that's the reason they're reworking them. Why then, were 2/3 of the items nerfed, and the last one getting a power neutral change? Let's look at the items individually, and then as a whole.
AD lowered from 60 to 55
Total cost increased from 2900 to 3000g.
Out of Combat Movement speed increased from 20 to 40.
Unique Active - movement speed changed from [20% movement speed] to [40% movement speed decaying down to 20% over the duration]
This item got a roughly power neutral or slightly power increased change.
* At the cost of 5 AD and 100 gold, we get 20 Out of Combat movement speed, (worth roughly 100 gold, based on the old Serated Dirk, or based on Mobility Boots) and a medium buff to the passive (double the movement speed).
* The passive movement speed is worth the extra cost, which means we're trading 5 AD for a medium buff to the passive. At best, this is a small buff to the item.
**Duskblade of Draktharr**
Build patch changed to [Serrated + Caufield's + XG]
Total Cost lowered from 3250 to 3000
Attack damage lowered from 65 to 55
Unique Passive- Nightstalker - proc damage changed from [75 + 200% lethality] true damage to [150 physical damage.]
[added] Unique Passive- Nightstalker proc now slows by 99% for .25 seconds.
[added] Now grants 10% cooldown reduction.
[removed] Out of Combat Movement speed lowered from 20 to 0
This item got a significant nerf in my opinion. First let's look at the easy stuff.
* The item costs 250 gold less, and gives 10 less AD. I'd say for a fully upgraded item, that's worth about the same.
* Then it got the movement speed passive removed, and a small slow (big percent, but only 0.25 seconds). I'd also consider this roughly the same. It also gained 10% cooldown reduction, which is pretty good.
* However, it took a huge hit to the unique passive. Even as a first item, the passive now deals less damage than it used to. With just Duskblade and no other lethality items, you have 45 lethality with lethality runes and masteries. That means the old passive did 165 true damage. The new one does 150 PHYSICAL damage, which ends up being roughly 100 damage in the end if the mid laner buys Seeker's Armguard, or closer to 140 damage if they don't or you're attacking a non-mid-lane squishy. Either way, it's a nerf.
* You do get CDR for it, but I would argue the CDR isn't worth it for reasons you'll see later. This isn't even considering that the old passive scaled into the late game, while the new one becomes almost worthless once the squishies get some armor and health from levels.
**Edge of Night**
Recipe changed to [Serrated + Pickaxe + Ruby Crystal + Gold]
Total Cost lowered from 3100 to 3000
[added] Now grants +250 health
[removed] Out of Combat Movement speed lowered from 20 to 0
[removed] Magic resist lowered to 35 to 0.
This one also looks like a (small) nerf to me.
* 35 magic resist is worth far far more than 250 health against magic damage, which most lethality users go up against (since most are played mid).
* It also lost the movement speed passive, while getting its cost lowered by 100, which as discussed before, is roughly neutral (though I would argue a fully built item should be more efficient than an item that costs 1k gold, so this is a small nerf too).
Finally, and most importantly, let's look at how the items work together to create item builds.
* Firstly, as a first item, Duskblade is now a viable alternative to Youmuu's. Probably even better with the cost reduction to Duskblade and the cost increase of Youmuu's. However, after this, everything goes wrong.
* Champions like Zed which want CDR as fast as possible can no longer build Black Cleaver as it prevents them from building Youmuu's because it would go over CDR cap with runes (every assassin takes 10% cdr runes). If you don't want to take CDR runes, you can only opt into defensive stats in Glyphs, as AD champs don't have an offensive alternative for Glyphs like AP champs do (other than CDR).
* Champions like Talon, which desperately need damage, no longer have high-damage options after first item Duskblade.
* Edge of Night is still a poor item to build early, even worse now because the defensive stat is less specialized so you're better off getting Black Cleaver. However, Black Cleaver can't be built with the other Lethality items, because again you run into the problem of going over CDR cap.
* On that note, there's almost no good 3rd item to give 10% CDR to buy along with Duskblade and Youmuu's, other than perhaps Maw and Death's Dance. Both are somewhat defensive, and while Death's Dance is a decent buy right now, that's only because it's somewhat overtuned.
All in all, builds were nerfed all around, as Edge of Night is a worse defensive option now, Black Cleaver is a weaker choice now, and you can't opt into a higher damage build because damage was nerfed across the board.
While 1 item spikes and maybe 2 item spikes were buffed, in my opinion assassins should not be forced to win the early game or fall behind as a role. I think assassins should have variety within the class too, as every other class has champs that are better early and champs that are better late.
As a final note, let's look at the changes when you buy all 3 lethality items:
-20 out of combat movement speed
-35 magic resist
Significantly (roughly 100 damage) weaker Duskblade passive
+10% cooldown reduction
Tiny slow on Duskblade
Medium increase to Youmuu's speed boost
Looks like at best a neutral change with the downside of being less item slot efficient, and all when Riot said lethality items were weak.