When another creature you control dies, transform this creature.
"With Avacyn gone, I no longer knew who to pray to. Alone, I wept for a sister who would never return."
(24/09/2021)
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.
(24/09/2021)
Multiple creatures dying will cause Bereaved Survivor to transform only once.
(24/09/2021)
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.
(24/09/2021)
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.
