Lucas had disappeared from the mansion while leaving Belle and the others back in the Adams mansion. It was still afternoon, and there was no time to go to the land of the dead if he needed to find a reaper. The Grims could pass in and out along with the death reapers, but it wasn't the same when it came to souls as they needed a medium to pass to the land of the dead.
He was in Valeria, searching for the couple out of which one of them, the girl was part of the alignment of stars similar to Belle. Keeping her alive was of utmost importance because if one died, then it would affect the rest and no one would be alive along with the possible destruction of the world where the mortals lived in.
