Deltarune - Ralsei can't be trusted

My theory is that as a Darkner native to the one Dark World that is SUPPOSED to exist, he has immunity/takes precedence. Kinda like he's the sysadmin of the Dark (He is the Prince after all). As to how he got there, he probably used the great door that we accessed the Card Kingdom through, since we now know Castle Town and the Card Kingdom are actually two separate Dark Worlds.