What happened to boots and 3 pots? Well the last patch increased based movement speed by 25 and reduced boots of speed by 25 so it doesnt impact early game as much as it used to. Take the flask for more sustain in lane and the ward to help keep you aware of the enemy jungler. The mana potion is taken over a health potion because you should be making good use of your passive. Next buy 1 or 2 Doran's rings as they will also help you with sustain in the laning phase before you can make your big purchases. Also Doran's Ring has a new ability that gives you 5 mana when you destroy an enemy unit so even if you have to play passively due to low mana you can get more mana if you just make sure you last hit! (which you should be doing anyway!)
We take haunting guise and sorcerer's boots boots for the flat magic penetration: gives good damage early game because not many people will be getting magic resist just yet which can lead to you doing nearly true damage.
We get Rylai's if we need some health and extra kiting tools.
We build our Haunting Guise into Liandry's Torment if they are getting tanky as the percent damage will help alot! If we have Rylai's we get the double effect of Liandry's due to the slow giving us even more damage to shred tanks. I'll Call it Rylai's Torment.
Deathfire Grasp was remade very well. We take it for the ability power, cool down reduction, and it's activate so we can take down targets fast.
Percent damage from Deathfire+Rylai's Torment+ your abilities= You Have Slain An Enemy.
NOTE: You do not have to follow this build purchase order or items. There are a lot of different ways to play and you should adjust your build accordingly each game. This is a general recommended build that works well if their team is super tanky or squishy. Buy the items you feel are right at the time!
If their AP carry is doing well get Athene's for the magic resist. You can also get Abyssal Scepter in place of this if you are able to your blue buff but if your in solo queue I'd suggest Athene's for the extra mana regeneration and the cool down reduction to substitute blue buff as your jungle may not always give you blue.
If their AD carry is doing well or they have a lot of AD get Zhonya's Hoursglass for the armor. It's activate ability can also save you in dire situations as you can Zhonya and then ult or to re-position yourself
Magic Penetration Option.
If your opponents have less than 100 Magic Resist get Abyssal Scepter( the extra magic resist is also good for your durability), otherwise get Void Staff as it will shred down their magic resistances better than Abyssal Scepter. Also take into consideration if you have a lot of AP on your team the Abyssal Scepter may help overall in team fights more so than Void Staff.