Spirit Harvest is an essence gem which causes players to receive more experience points but to no longer receive any power-ups from killing ghosts. It has a counterpart in the form of Absolute Dedication, which does the same for human opponents. It can be obtained in the Necropolis, being found in a ceramic urn within the Tomb of the Dignified Collector.
In-game description: "Those who hunt ghosts and wayward spirits often find that the knowledge gained from defeating ephemeral foes is more useful than the recuperative pockets of essence that they leave behind."