The best way to do that is to set nap.bunrieucua.com as an addon domain, not an alias, and then put 301 redirects on ygo.helioho.st so everyone who goes to the old URL will get a permanent redirect to the new domain. This is part of the reason I try to talk people out of setting domains up as aliases. This is also the reason I didn't want to waste my time adding it as an alias for you, and waited for Wolstech to do it instead. Now that it's already added as an alias it would be an even more waste of our time to remove it and re-add it as an addon.
I'm just typing all of this for anyone know who searches and finds this. The only reason to add a domain as an alias is if you're planning on using both domains forever, and are not retiring either. For instance, I have krydos.com. If I wanted to add krydos.net to have exactly the same content and functionality as krydos.com, and I wasn't going to redirect anyone from one domain to the other, it would make sense to add it as an alias. Maybe the documentation on the wiki can be improved to help people realize that they probably don't want an alias.