The Nonentity Relic?

At the beginning of your upkeep, choose a random type of mana that has not been chosen before.
Mana of types that has been chosen can't be added to players mana pools nor be used to cast spells or activate abilities.
From the weakest of spellcasters to the most powerful of gods and planeswalkers, they all share the same source of power and thus the same weakness.