The sea clan (part 1)

Before leaving the settlement, Daimon used Disaster to cut a literal room of the castle to then store it in his inventory, the black material of which the castle was made, had only one property which was stabilize nether.

Apparently the four races had their own means to deal with the nether that was in constant clash with the different kinds of mana on the atmosphere, the Black Night race used this mineral which only they knew how to process, to do that.

As to why Daimon only took a part of it, was because Ashelia, the first of the members of the Black Night race to surrender, informed him that the castles of the higher ranked tribes were made of a better-quality material, so he just took a "sample", who knows when something can come in handy and if not, he knows a certain fox girl who'll be more than happy to study a new material.