One who slanders or defames the name or reputation of another person.(noun)
Wiktionary.org : Text is available under the Creative Commons Attribution-ShareAlike License
Use "slanderer" in a sentence
"A slanderer is one whose tongue is set on fire of hell; so much, and so directly, do these do the devil's work, that for it the devil's name is given to such."
"That the slanderer was a fellow writer makes it even more heinous."
"The slanderer was a Mr. Prendergast, who affirmed that Dr. Carey's conduct had changed so much for the worse since the departure of Lord"