[@Shorticus]That's something I'm working as we speak, rewriting some sections of my NS, Hallows included.

As of now, the most set in stone method is when a necromancer gets lucky and instead of just filling a body with souls and spirits until it comes to life, they some how manage to obtain the full, undamaged soul of the original soul of the body. Of course, this is the rarest way.

As of right now, I'm debating if I should have it be like:

1) Dark Souls where there is an Undeath Plague that kills people with only some returning as living Hallows instead of Feral Hallows who are basically zombies.

2) Its most likely going to be those who have a very strong or a very weak attachment to life and desire to live to the point of extremes which would force their souls back into their bodies and follow the beckoning bell of Myrstrost. There are more details to this of course but I'm cutting it down for the sake of time and reading (I think its also the one listed in my NS).

At any rate, Hallowing works something like a Magical Disease, a mix of an illness and a curse which those affected are almost always treated the same as undead since they are basically undead, with scorn and disgust. I'm also only Hallowing races which their creators/owners have given me permissions to do so outside the common fantasy races; basically only "mortal races", I classify Dragons as something above being mere simple mortal beings so they can't be Hallowed at least by nature.