So, I read this series once in middle school. It may be trash, but I vaguely recall enjoying it. I can't find anything about it online. I will post the distinguishing factors I can remember.
1) It had a classic fantasy novel type cover.
2) It was as trilogy, I believe. At least a two-book series.
3) The main character's name started with a Q. I think it was Quenten (some spelling or another) but I'm not certain.
4) The first book is about him being something other than king. I want to say he wasn't royalty at all. He goes on an adventure with some amigos and ends up with an injured arm, but finds a glowing stone in a pool of water while hiding in a cave. The stone makes his arm feel better and eventually is forged into a sword. Yes, very King Arthurian.
5) The novel was older? I want to say it was probably published in the mid 20th century, but that's a guess.
6) There's a "tied upon a wheel" torture type scene in it... I think the school librarians didn't read some of the donations too closely.
I can't recall much about it, such as any words in the title. I hope that this is enough information to jog someone's memory. I don't believe it is a very well known book series, but I thought it was worth a shot.