It was the feeling of a bond, like that to his mate, being formed between him and the underworld. The missing link, the confirmation of the incredible secret he'd learned in the short time he'd been in the sacred closet.
The underworld had chosen its master and was officially, eagerly welcoming him.
Just like Darkyn said. He didn't want to guess if the Dark One had purposely told him or guessed right. With knowledge from the time-before-time, there was a chance Darkyn knew.
The only thing he was concerned about: letting the souls speak.