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.

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.

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.

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.

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