Eternity Passive Works in URF
Despite URF clearly stating that mana costs are reduced by 100%, 
and its upgrades calculate health restoration when using an ability based on its listed mana cost, rather than 0.
3 Comments
Things actually use mana. It's just that your mana doesn't go down. That's why tear items still work, and champs with skills that increase a mana cost per second or something can actually stop working (when the cost becomes greater then your entire mana pool)