Jacob got many things from the Soulless Ghost King, not just his appearance and prowess but also many treasures and, most importantly, his memories.
Although Jacob didn't want to do it initially, he had to admit it was an unexpected windfall. The biggest haul was not Soulless Ghost King treasures but the information about three types of Law Essences!
The first Law Essence was the Law Essence of Death in the Valley of Sunless Dusk, which Jacob already had claimed, so this information was useless. But it surprised Jacob that Soulless Ghost King was aware of this information.
Although he didn't know how much Death Law Essence was present in the Necropolis of Shades, he figured out that the Curse of Sunless Dusk would undoubtedly produce it.
