Thryx, the Sudden Storm

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

A spell or ability that counters spells can still target spells you control with mana value 5 or greater. When that spell or ability resolves, your spell won't be countered, but any additional effects of the countering spell or ability will still happen.

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 Thryx). The mana value of the spell remains unchanged, no matter what the total cost to cast it was.