Why not call someone what they want to be called? It ain't new. Just like it's polite to ask someone "can I call you x" or "do you prefer x or y" when you start to call someone a nickname or more personal name, someone can ask to be called x, and it's polite to do so. Names are arbitrary things, but at the same time often deeply meaningful to people.

My guess is that Kalcifer sounds cooler than their birth name? I think it sounds cool at least.

They might be trans and this is their first foray into explaining how they want to identify. Autism and gender issue have a huge co-diagnosis.

I'm a millennial and changed my name. I don't know why your relative wants to be called something new. There are thousands of possible reasons. You could ask them directly. If they're autistic, there's a very good chance they'll be happy to tell you their thought process in direct and concrete terms. You could also attempt to compromise by calling them "Cal" which is a more normal name. Denying them without understanding is pretty much the worst move >_>

They played too much WoW.

No, but seriously, calling people whatever they like is OK. As another Millenial, at least in my corner, we all had some kind of nicks and they came to real life. I have a lot of friends who still go by their nicknames from decades ago. I also changed my own name a little and am currently almost exclusively known by that name. I don't go around telling people why but my name is connected to trauma quite strongly. I can even go further in my family history to have an example of how people have been doing that always. My grandma was called by two names. She moved to the city and decided to go by another name. I was a little bit confused as a kid when my great-aunt called and asked her by a name I had never heard before. I might understand not calling someone something offensive, but Kalcifer is quite mild. In the end, it doesn't hurt anyone and is a tool for building your identity.