What about an Odyn's Veil for Summoner's Rift, but it stored physical damage instead?
Archaic Veil - 3200 Gold
+80 Ability Power +30 Armor +350 Mana
Unique Passive: Reduces and Stores 10% of Physical Damage taken. Max 500 Damage Stored.
Unique Active: Your next spell deals additional magic damage on hit equal to 50% of the stored damage. If 500 damage is stored, the spell will also slow briefly.
Builds from 
+ 
+
+ 1300 Gold
, but much better passive+active. I'll always get it as sup item.