Afterward, a number of treatises on sea serpent morphology were swiftly revised.

Tolarian Terror

This spell costs {1} less to cast for each instant and sorcery card in your graveyard.
Ward {2} (Whenever this creature becomes the target of a spell or ability an opponent controls, counter it unless that player pays {2}.)

The List

Dominaria United

To determine the total cost of a spell, start with the mana cost or alternative cost you’re paying, add any cost increases, then apply any cost reductions (such as that of Tolarian Terror). The mana value of the spell is determined by only its mana cost, no matter what the total cost to cast that spell was.

Tolarian Terror’s first ability can’t reduce the {U} in its cost.