Technically he was. He apprenticed under the spirit of Naga Sadow on Yavin 4. You may be thinking of Ludo Kressh.
No I was thinking of Freedon Nadd. Most of what I've read about him says that he awakened Naga Sadow from stasis on Yavin 4 and then later killed him after he learned the secrets of the Sith from him. I don't think I'd ever heard about him learning from a holocron of King Adas. In fact, since King Adas was a Sith ruler long before the arrival of the Dark Jedi Exiles who became the Dark Lords of the Sith, how would he even have had a holocron? I thought they were Jedi teaching equipment.