Longevity; AP Mage Item!
Currently working on Balancing it.
First Version of Longevity;
----Longevity-----
80 Ability Power 20% Cooldown
Passive: Reduce damage by (10%-2.5%) [Based off of mana pool] (100% mana = 10% damage reduction, below 40% mana = 2.5% damage reduction). Per Point of Health Lost Prevented; Triple the Mana is lost instead. Only to Champion Damage. Heals and Shields are reduced by 10%. Must have mana in order to reduce damage.
Build path= Fiendish Codex + Fiendish Codex + Amp. Tome
Cost = 900 + 900 + 435 + 965 = 3200 Cost.
Second Version of Longevity;
Active: For 4.5 seconds reduce all incoming damage by 10%; While in this state movespeed is reduced by 10%, Heals and Shields are reduced by 10%, and Mana Cost are all doubled in cost. Must have Mana to use.
Understanding the stats and passives.
Stats; as it currently stands in League of legends. There is 3 Go to items for every mage to start.

But generally speaking;
and
have a worse payout for at least 10 minutes of the game.
So this is an item to help those mages that want to rush
or
.
80 Seems like the fair number to give an AP item.
20% Cooldown to compete with Morellos as a secondary item.
PASSIVE:
Damage Reduction 10-2.5%. 10% damage reduction on this item makes the item 4,000 Gold in value.
Only accounting for MAX potential; (Looking at your Pro Players).
Clearly an item that at best is worth 4,000 costing 3,200 needs SOME drawbacks.
The primary drawback for this item is the mana lose based off of the damage taken. Therefore when you get attacked you also lose the effectiveness of the item itself.
To counter-act those champions that could abuse this item for
First Version: Reduced healing and shielding for Ryze, Cass, Mao, and Singed.
To prevent just tanking baron and towers forever; it is only applied to champion damage.
It also is not usable by champions that do not have Mana. Meaning it kills off some of the other champions that do not have to be concerned with Mana.
Second Version:
Because it is an active during that time the champion needs a clear weakness; so being able to just simply walk out of his or her range makes it very weak.
Same reason with the heals and shields.