Jacob has already more or less guessed the unique situation of the Ghost Race. Basically, the Soul Abyss had been connected to the Divine Soulless Pagoda or at least created by it, and he was the main reason the Ghost Race was able to become a legendary king-rank race.
Although Jacob had no idea about the more profound mystery of the Soul Abyss, he was sure that Soulless knew everything. So, when Jacob captured the Divine Soulless Pagoda, everything connected to it instantly crumbled, including the Soul Abyss, Soulless Palace, and who knows what else.
But there was another thing Jacob was curious about as he asked Alma, "Tell me, why couldn't you leave the Soulless Palace or, as a matter of fact, the Ghost Domain even when the Soul Abyss wasn't active? I can only assume that the reason you were unwilling to have a child with the Soulless Ghost King should be because of your unique situation?"
