1. The slow, ominous ringing of a bell often used to indicate a sad event; such as, a funeral: “I heard the church bell knell which hinted that somebody had died.”
2. A phrase used to signify the end of something: “The failure of the rains proved death knell for crops in the area.”
Knoll, on the other hand, is a small gently rounded hill: “While standing at the top of the knoll, I could see the entire valley.”