Thunderkin Awakener — ruling 1

The target card must have lesser toughness both as Thunderkin Awakener attacks and as its triggered ability resolves. Other abilities that trigger whenever Thunderkin Awakener attacks can’t raise its toughness in time to allow the ability to target cards with greater toughness.

Thunderkin Awakener — ruling 2

If Thunderkin Awakener leaves the battlefield after it has attacked but before its triggered ability resolves, use its toughness as it last existed on the battlefield to determine whether the card is still a legal target.

Thunderkin Awakener — ruling 3

You choose which player or planeswalker the returned creature is attacking. It doesn’t have to be attacking the same player or planeswalker that Thunderkin Awakener is attacking.

Thunderkin Awakener — ruling 4

Although the returned creature is an attacking creature, it was never declared as an attacking creature. This means that abilities that trigger whenever a creature attacks (such as that of Thunderkin Awakener itself) won’t trigger when it enters the battlefield attacking.

Thunderkin Awakener — ruling 5

If the new creature leaves the battlefield before the end step, that card remains in its current zone. It won’t be sacrificed.

Thunderkin Awakener : règles, fonctionnement et interactions (MTG)

Thunderkin Awakener 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

Haste Whenever this creature attacks, choose target Elemental creature card in your graveyard with toughness less than this creature's toughness. Return that card to the battlefield tapped and attacking. Sacrifice it at the beginning of the next end step.

How it works : Thunderkin Awakener

Original English text — Wizards of the Coast / Scryfall

#1July 12, 2019· Wizards of the Coast
The target card must have lesser toughness both as Thunderkin Awakener attacks and as its triggered ability resolves. Other abilities that trigger whenever Thunderkin Awakener attacks can’t raise its toughness in time to allow the ability to target cards with greater toughness.
No other card is named in this ruling.
#2July 12, 2019· Wizards of the Coast
If Thunderkin Awakener leaves the battlefield after it has attacked but before its triggered ability resolves, use its toughness as it last existed on the battlefield to determine whether the card is still a legal target.
No other card is named in this ruling.
#3July 12, 2019· Wizards of the Coast
You choose which player or planeswalker the returned creature is attacking. It doesn’t have to be attacking the same player or planeswalker that Thunderkin Awakener is attacking.
No other card is named in this ruling.
#4July 12, 2019· Wizards of the Coast
Although the returned creature is an attacking creature, it was never declared as an attacking creature. This means that abilities that trigger whenever a creature attacks (such as that of Thunderkin Awakener itself) won’t trigger when it enters the battlefield attacking.
No other card is named in this ruling.
#5July 12, 2019· Wizards of the Coast
If the new creature leaves the battlefield before the end step, that card remains in its current zone. It won’t be sacrificed.
No other card is named in this ruling.