Ahhh, a new item for mages, Athene's Unholy Grail. This upgrade solves a legitimate problem for mages in that most have to either have constant blue buff, thereby robbing an important creep for gold income and a decent amount of experience from your jungler not to mention the benefits that the buff itself gives.
With this item, Chalice of Harmony has the potential of becoming a core item for the early game, which really never happens except for a very few select exceptions (new Anivia players). Which is great, because not only does it free up that all important blue buff for your jungler, but it offers you resistances to help you survive better in the middle lane.
Yet another effect of this item perhaps may be the revival of Fiendish Codex as a viable early game item. This used to be amazing because with it, you had the option to build into a Morello's Evil Tome for a more sustained build, or go into a more offensive build with Deathfire Grasp. But, since they replaced Fiendish Codex with Blasting Wand in the Deathfire Grasp recipe, the only reason that you would actually build a Fiendish Codex would be if you had the intention of upgrading it to a Morello's Evil Tome in the mid/late game, which is a shame because early game AP, cooldown reduction, and mana regeneration was amazing.
Now, Athene's Unholy Grail gives AP, cooldown reduction, magic resistance, and massive mana regeneration. Normally, all of these offensive aspects and the utility cooldown reduction and mana regeneration were handled by the item Morello's Evil Tome.
Morello's Evil Tome costs 2330 gold and gives 75 AP, 12 mana regeneration, and 20% cooldown reduction. In effect, this means that you are paying about 31 gold per AP, 194 gold per mana regenerated, and 116.5 gold per 1% of CDR.
Athene's Unholy Grail costs 2950 gold and gives 80 AP, 36 magic resistance, 15 mana regeneration stacked with its unique passives of 1% per 1% mana missing and 12% regeneration upon a kill/assist, and 15% cooldown reduction. This totals out to 37 gold per AP, 199 gold per 1% CDR, 82 gold per point of magic resistance, and a variable mana regeneration rate that could very well drive the cost of that aspect of the item to free.
So, with a cost increase of 21%, you get an item that gives everything that Morello's Evil Tome gives, plus defense. From a flat numbers perspective, the equivalent option would be to get a Morello's Evil Tome for the mana regeneration, AP, and cooldown reduction and then purchase a Negatron Cloak to get your magic resistance, bringing you to a total expenditure of 3070 gold.
The one saving grace of Morello's Evil Tome (and I know that you've probably been yelling at the computer while reading this the whole time thinking that I forgot it) is its active that applies a Grievous Wounds effect. The fact that this item does have an active in addition to its impressive passive abilities makes the argument that it is in fact one of the most cost efficient items in the entire game.
But to actually compare the 2 items, the limits of this particular active in the game must be analyzed. There are a few champions where healing/regeneration damage will cripple them ( Fiddlesticks), but this seems more situational that an every game necessity and if Grievous Wounds is a necessary part of the game, than there is a free alternative that is extremely common and doesn't hinder the gameplay capabilities of really any champion, Ignite.
The above just represents my thoughts and feelings about Athene's Unholy Grail. They may be true, or dead wrong, but I figured that it might be an interesting discussion and I felt like typing (idle hands and all of that jazz). So if you read all of that, I apologize for the lost IQ and puddle of drool on your computer. Good luck on the Rift and Scar