So, given various hints, we can guess that expanding Norse cultivation will involve an act of sacrifice, an ordeal of some sort. It seems likely this is how Charred-Soul came about - the pain of losing the flesh of one's hand, but I wouldn't be surprised if the Nine Steelfather Pileup was part of it, too. It might be that Hallr did it intentionally, and has forgotten that he did so as part of being spread across 20 descendants - it might also have been an accident.
It seems increasingly likely to me that the sacrifice of the potential for long-life is powering Norse cultivation. Christian cultivation is bound by the sacrifices of much of the population to the priests, prayers, and possibly other things - oaths, acts of service, years of intense training. If Steelfathers are really something other, then they likely sacrifice something too, perhaps related to the Curse of Steel, perhaps not.
The idea of creating an inner soul-space, a runestone/dantien etc, suggests to me that we would need to undergo a symbolic mortification of the flesh. Tattoos probably wouldn't do it, but possibly branding or scarification. Maybe even disembowelment, or a blood eagle, since we're literally creating a space inside us and symbolically, that might involve taking stuff... out. It would have to be intense, because no one else has gotten this, and obviously I'm not suggesting random mutilation - we would want to be following some inspiration, some advice from some spirit or divine being. Odin's sacrifices were to someone - Mimir, and the runes themselves.
