Justice may be blind, but Lavinia is always watching.

Lavinia, Azorius Renegade

Each opponent can't cast noncreature spells with mana value 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.

Secret Lair Drop

"I told Jace that Ravnica would fall apart if he didn't take his responsibilities seriously. I didn't want to be right."

Mystery Booster 2

Ravnica Remastered

Effects that modify or replace the cost to cast a spell (such as flashback or casting a spell without paying its mana cost due to suspend) don't affect the spell's mana value, so they won't change whether Lavinia's first ability restricts that spell from being cast.

For spells with {X} in their mana costs, use the value chosen for X to determine the spell's mana value.

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. However, if that spell also has additional costs that require mana, paying that mana will stop Lavinia's last ability from triggering.

Players may cast spells that they know Lavinia's second ability 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.