TIL you lose 5 AP when you upgrade Fiendish Codex into Athene's Unholy Grail
... if you don't take Athene's passive into account.
Fiendish Codex: 35 AP Athene's Unholy Grail: 30 AP
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So don’t buy it on manaless champions, and you won’t have to worry about losing AP on upgrade. Everyone with mana who upgrades Fiendish into Athene’s gains AP.
Athene's is one of the best items when bought with these: 
You get great value if you buy those items with Athene's. The more you buy the better it gets. Not buying Athene's on an Enchanter with those items is just wasted potential AP.
thats a bit annoying
you will always have a net AP gain due to the passive though.
Harmony item passive also becomes the Dissonance item passive. So you also lose out on potential health regeneration in exchange for AP.
A lot of items do this since their passives grant better potential and effects when the item's finished.
Before the Mid Season Mage Update in like 2016? I forget when exactly. Catalyst the Protector used to grant HP and Mana restoration on level up. But when upgraded to Rod of Ages, you lost that passive effect in exchange for stats per level.
Unless manaless, Athene's should give 50 AP.
lmao that's actually wild asf