The line seems to be that sentient humanoid beings have black souls.' And now I think I've figured out Mannimarco's thesis presentation that got him reprimanded by the Psijic Order. I suspect the real in-game lore reason is 'We tried using various levels of soul gems on different creatures and people and these were the highest level soul gems that worked on each. Dead NPCs such as Olaf One-Eye and Dragon Priests have grand souls, where you would expect them to have black souls in life. The farmer is unquestionably a sentient humanoid (an NPC) and so has a black soul, and yet the giant, not being at that level but still very powerful among the non-NPC class, has a greater soul. As with the falmer, you'll see different levels of souls according to their level in game, but none of them ever have the sentience to reach the level of sentient humanoid being except for certain Dawnguard NPCs. In meta terms, a black soul gem is used to capture the soul of an NPC, which thanks to the coding is men, mer, beastfolk, and dremora.ДЄs for power, it appears to be related to level. So unfortunately, on a meta level, TES III Morrowind isn't much help since the black soul gem wasn't developed as a game item yet.Д«oth Skyrim and Oblivion seem to use black soul gems to capture sentient, humanoid beings, including some types of higher daedra like dremora or golden saints. There's a similar issue for the NPCs one can soul trap, since we would expect them to have black souls in later games. So its hard to judge whether or not certain things in Morrowind like the Dagoths might have had black souls or kept the grand souls they had in game. The matter is complicated by the fact that black soul gems only appeared in games after TES IV: Oblivion where they were introduced as part of Mannimarco's necromancy. In the case of the giants, while they do use tools/have a rudimentary sort of civilization, we don't see anything more than that. In the case of the Falmer, they are corrupted and betrayed Snow Elves (see Knight Paladin Gelebor's description of what the Dwemer did to the Falmer: ) who used to have black souls until the Dwemer betrayed them and did something that made their souls white so the Dwemer could use their souls for their machines. It does indeed appear to be a certain level of intelligence.

Skyrim black soul gem vs grand