Cost increased, ability power decreased.
Oblivion Orb is a ubiquitous mage pick-up due to the ability to stack its magic penetration with Sorcerer's Shoes relatively early in the game. That being the case, why didn't we nerf the magic penetration? We're shrinking Oblivion Orb's contribution toward ability ratios so it's less attractive for late-scaling mages while still satisfying to rush for champions with high base damages or are looking for an early power spike before moving onto main-build items.
COST :  ⇒ 1600 gold
ABILITY POWER :  ⇒ 20
Because flat magic penetration is cool, just not for the Liandry's Torment line.
BUILD PATH : Ruby Crystal + Amplifying Tome + 665 gold
TOTAL COST : 1500 gold
ABILITY POWER : 25
HEALTH : 200
UNIQUE PASSIVE : Touch of Death: +15 flat magic penetration