"It was Sigarda who lifted me from despair and gave me the strength to fight on."

Bereaved Survivor // Dauntless Avenger

Whenever Dauntless Avenger attacks, return target creature card with mana value 2 or less from your graveyard to the battlefield tapped and attacking.

"With Avacyn gone, I no longer knew who to pray to. Alone, I wept for a sister who would never return."

When another creature you control dies, transform Bereaved Survivor.

Innistrad: Double Feature

Innistrad: Midnight Hunt

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 won't trigger when it enters the battlefield attacking.

Effects that say that the returned creature can't attack (if the creature has defender, for example) affect only the declaration of attackers. They won't stop the returned creature from entering the battlefield attacking.

Multiple creatures dying will cause Bereaved Survivor to transform only once.

You choose which player or planeswalker the returned creature is attacking. It doesn't have to be attacking the same player or planeswalker Dauntless Avenger is attacking.