Thryx, the Sudden Storm
Flash (You may cast this spell any time you could cast an instant.) Flying Spells you cast with mana value 5 or greater cost {1} less to cast and can't be countered.
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Official rulings (6) · Scryfall
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.
Players can't attempt to remove Thryx to raise the cost of a spell you wish to cast, but they can remove it to allow their spells and abilities to counter your spells.
Thryx's last ability doesn't apply to itself while it's a spell.
If Thryx enters the battlefield while a spell with mana value 5 or greater that you cast is on the stack, that spell can't be countered, even though you don't get the {1} discount.