This was pre C3V/SoV. Basically someone would make a customer, someone would nominate, there was 2 weeks of discussion, then six Judges would have to vote with minimum of 4, preferably 5-6 YES votes. See
HALLS OF VALHALLA (Best Of Custom Units) - Nominate Here!.
If you want to still play them, go for it! I don't think hardly any "sanctioned" tournaments (Online, regional) have allowed them in years, but some used to before SoV/C3V. Play at your own risk as since they are so old designs, that both later waves of official units as well as SoV/C3V units may "break" them, plus they were designed with older meta of less points, more range-friendly maps, no large bonding armies, etc. But I am sure some of them are a blast. I think I played by Blix along with older HorrorClix based customs, and remember they were fun.