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1. We will see ;).
2. Darius's power is effectively, complete and total invulnerability. No outside influences can affect his body, but his normal biological functions still work so he can still age, gain weight, grow fur, get tired, etc. How everything works will be explained later on.
3. Vistella is actually a bit stunted technology-wise due to an event that we will learn about in Build 2.
Their tech is effectively split into 2 branches, mundane and magical. The mundane branch is currently at about the same level as our tech in the 20th century with things like widely available electricity, lights, early radios, and a very advanced printing industry (yet, no cameras and movies). They also have access to some fantastical inventions from their magical branch, like universal translators, teleportation circles, nuclear fusion reactors, artificial wombs and bio-prosthetics. These 2 branches aren't entirely separate, and often contribute to each other's growth.
Quantum physics aren't something they have grasp on yet, but their study of the magic system has lead to a very similar field.