Thundermaw Hellkite — ruling 1

Thundermaw Hellkite will deal 1 damage to each creature with flying your opponents control whether those creatures are tapped or untapped.

Thundermaw Hellkite — ruling 2

If the damage that would be dealt to a creature with flying is prevented, that creature will still be tapped. If that damage is redirected to another creature, the creature with flying will be tapped, not necessarily the creature that ultimately was dealt damage.

Thundermaw Hellkite : règles, fonctionnement et interactions (MTG)

Thundermaw Hellkite 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

Flying Haste (This creature can attack and as soon as it comes under your control.) When this creature enters, it deals 1 damage to each creature with flying your opponents control. Tap those creatures.

How it works : Thundermaw Hellkite

Original English text — Wizards of the Coast / Scryfall

#1November 17, 2017· Wizards of the Coast
Thundermaw Hellkite will deal 1 damage to each creature with flying your opponents control whether those creatures are tapped or untapped.
No other card is named in this ruling.
#2November 17, 2017· Wizards of the Coast
If the damage that would be dealt to a creature with flying is prevented, that creature will still be tapped. If that damage is redirected to another creature, the creature with flying will be tapped, not necessarily the creature that ultimately was dealt damage.
No other card is named in this ruling.