Sorderon's Shadow.
After the amazing Lords Of Midnight and Doomdark's Revenge, this was released some time later by Beyond as some link to the world that had been created by the late great Mike Singleton. It was so obtuse that I didn't have a clue what I was doing, where I was meant to go or why.
Maybe there was a great game in there but I never found it. Perhaps the disappointment is why I remember it as the 'worst' for me.