This is a theory I've held onto since 2012, originally based on graphical clues throughout the game (most significantly the updated Ritual Circle model). The release of Ports only expanded the concept, and I've alluded to it in many other threads (including a lore for Temekel and a quest series proposal). With the interest in Eastern Lands lore and Rowl*y's promise to focus on narrative, I thought this topic merited discussion.
Magic in the Wushanko Isles does not function the same way it does in the West. With little access to runestones, mystics of the east wishing to manipulate anima have turned to other means - siphoning it directly from creatures' life essence.
In its more mundane forms, this art is known as soothsaying, and can involve the sacrifice of small animals or working with the entrails of larger beasts. Soothsayers are closer to the seers of the West, having limited clairvoyant powers, and able to subtly influence fortunes, particularly upon the sea.
But in its more extreme forms, Occultism seems to harness almost limitless power, its practitioners able to summon hordes of powerful demons, subjugate the wills of others, even obtain immortality. Occultists work with far more sinister catalysts, preferring great sea monsters (Kami, creatures of pure anima), or the sacrifice of intelligent creatures, including humans and sea orphans.
This type of magic is not necessarily unheard of in the west. Nomad practiced a modified from of anima siphoning. One of the 12 Elder Artefacts seems to exist specifically for that purpose. But, three instances seem particularly compelling: