Guest of an angel

When Tirna woke up, she had no idea where she was. Everything was unfamiliar, even the gravity. And the huge, imposing, winged being looking down at her scared the senses out of her.
"I an Boel," the grayish gigant was saying, his deep bass rolling like thunder. "One of the lost archangels. And I have called you to me, Tirna Caperi, because I need your help."

'My Help', Tirna couldn't help thinking. Why would such an obviously powerful and imposing figure need her help? And who were really 'the lost archangels'?