Obscure one, but Raymond E Feists Midkenia novels, which he's been releasing on and off for 25 years now. They started off as seminal 80s fantasy books, and he even co-wrote a trilogy set in that universe with Janny Wurts (the Empire Series) which is probably the best thing either of them wrote. But they've become so bad they are unreadable. Every new book is worsely written, every new book has to try and top the giant revelation from the previous book, and now they read more and more like the script of a bad computer game or rpg session.
Was telling really when he published the remastered version of the first novel, Magician, and it was worse than the original text as he'd added several thousand words in. Less is more sometimes.