Lavinia, Azorius Renegade
Legendary Creature — Human Soldier
Each opponent can't cast noncreature spells with converted mana cost greater than the number of lands that player controls.
Whenever an opponent casts a spell, if no mana was spent to cast it, counter that spell.
"I told Jace that Ravnica would fall apart if he didn't take his responsibilities seriously. I didn't want to be right."
Illustrated by
Ravnica Allegiance Promos#189sRareEnglishLegality
Rulings
Players may cast spells that they know Lavinia will counter. Any abilities that trigger when spells are cast will trigger and resolve if appropriate, and any effects that count spells cast will count those spells if appropriate.
Effects that modify or replace the cost to cast a spell (such as spectacle) don’t affect the spell’s converted mana cost, so they won’t change whether Lavinia’s first ability restricts that spell from being cast.
For spells with 
in their mana costs, use the value chosen for X to determine the spell’s converted mana cost.
If an effect allows a player to cast a spell without paying its mana cost, that player can’t choose to cast it and pay its mana cost unless another rule or effect allows that player to cast it that way.