Reading the latest chapter felt like this:

Chihiro's mom (or even his dad) is probably from the Island, right? It's the only way the swords could get forged, and the ability passed down to Chihiro for sure. In an earlier chapter, Enten was described as an ornamental sword, so he probably had to infused it with Datenseki after it'd already been finished.
Someone brought up Char is probably from the island as well, seeing as Sojo wanted to use her to stabilize the datenseki. I wonder if we'll get to learn about any other kinds of powers originating from the Island, seeing as these two aren't really powers to attack, more like resistance magic. (It also fits nicely with the idea that Japan is the attacking nation... they have to resist them...)