Read it official: VIZ or MANGA Plus. Chapter 14 is the issue the technique catalog keeps pointing at. True Realm (本領, Honryō) is the brief, meant: the blade when the person finally means it. Sojo’s Cloud Gouger “gains slaughter.” Enten’s True Realm, named later, is Magatsumi’s death. Same word. Different lock.
What the issue is not
It is not a power-up montage. The Anti-Cloud Gouger Special Forces have already noted that the Enchanted Blades have more in them than the present-tense wielders are spending. Chapter 14 is the book agreeing, then showing what “more” costs when the customer is Sojo.
Dark power (Kuroi chikara) is the later, blacker cousin: the lock at the edge of death, or when the blade itself is dying. Mei: Shred. Suzaku’s black flames. Magatsumi’s ordinary darkness. File: the essay, technique catalog, technique index.