Tolarian Terror
Creature — Serpent
This spell costs 
less to cast for each instant and sorcery card in your graveyard.
Ward 
(Whenever this creature becomes the target of a spell or ability an opponent controls, counter it unless that player pays .)
Afterward, a number of treatises on sea serpent morphology were swiftly revised.
Vincent Christiaens
Dominaria United (DMU)
• #72Rulings
Tolarian Terror's first ability can't reduce its cost below {U}.
(wotc • 2024-11-08)
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.
(wotc • 2024-11-08)
Rulings
Tolarian Terror's first ability can't reduce its cost below {U}.
(wotc • 2024-11-08)
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.
(wotc • 2024-11-08)