Ryujin
Legend
You might be thinking of the Student of Caiphon paragon path, which was errata-ed to be more restricted to warlock abilities rather than being a goto for anyone who did radiant damage.
No, it's definitely Radiant one that I'm thinking of. When it first came out, in Dragon, it said "21st level." Later on, as my own character approached Epic, I was looking for an appropriate ED for my Fey/Darklock, who I had styled as becoming a Prince of the Summer Court. Originally I looked at Prince of Hell, asking if I could reflavour it for a Fey by making the summoned demon minions into some sort of Fey creature. The DM said no. Then I looked at Radiant One, which fit my character concept while simultaneously improving my damage to the point where I would be close to the other party Strikers, but he said no to that too. When I looked it up at the time, it had been the subject of errata to make it a Warlock only ED. Now, everywhere I look, it's back to just "21st level."