User blog comment:ViridianFan/A few item ideas/suggestions./@comment-1330314-20150628190815/@comment-26541414-20150628204019

Countering mana-stacking isn't the only thing this item is good for, even though it deals 1000 magic damage on 5000-mana champions. It's also just good for dealing sustained damage, even with low-mana champions or no-mana champions. About the two-sided (non)benefit:

If they have enough mana to cast spells, it's not a big worry - you still deal quite a bit of their health in magic damage as their mana pool has been reduced by 20% - and that damage is dealt in magic damage to the target. And even then, 7.5% of the mana depleted will be dealt further in damage, and mind you, that is total mana, not current. So 7.5% of 5000 mana would be 375 damage every 0.25 seconds.

And about the "why would you counter mana stacking"? Well, it's because most items already focus around countering health/resistance stacking, so a mana-stacking item would be a refreshment from the items we already have. For example, you have Void Staff, Black Cleaver, etc. and Sage's Ring would be the first item of it's kind. Plus, as I said before, it's a pretty versatile item as it still deals a good deal of damage against low-mana or no-mana champions.

On another note, I had the intention on giving the item a 15% damage increase on the afflicted champion for 2.5 or 3 seconds, but since that looked too much like Deathfire Grasp, I decided not to include it. However, this is just my opinion, and maybe it would be a good idea.