Thryx, the Sudden Storm — ruling 1

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.

Thryx, the Sudden Storm — ruling 2

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

Thryx, the Sudden Storm — ruling 3

Thryx's last ability doesn't apply to itself while it's a spell.

Thryx, the Sudden Storm — ruling 4

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.

Thryx, the Sudden Storm — ruling 5

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.

Thryx, the Sudden Storm — ruling 6

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, the Sudden Storm : règles, fonctionnement et interactions (MTG)

Thryx, the Sudden Storm is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

Flash (You may cast this spell any time you could cast an instant.) Flying Spells you cast with mana value 5 or greater cost less to cast and can't be countered.

How it works : Thryx, the Sudden Storm

Original English text — Wizards of the Coast / Scryfall

#1January 24, 2020· Wizards of the Coast
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.
No other card is named in this ruling.
#2January 24, 2020· Wizards of the Coast
For spells with in their mana costs, use the value chosen for X to determine the spell's mana value.
No other card is named in this ruling.
#3January 24, 2020· Wizards of the Coast
Thryx's last ability doesn't apply to itself while it's a spell.
No other card is named in this ruling.
#4January 24, 2020· Wizards of the Coast
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.
No other card is named in this ruling.
#5January 24, 2020· Wizards of the Coast
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 discount.
No other card is named in this ruling.
#6January 24, 2020· Wizards of the Coast
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.
No other card is named in this ruling.