Word or expression for someone who intentionally says false statements to bring someone else down?

Someone who intentionally says false statements and/or facts about someone else (directly related to and/or about them) with whom that person is in a conversation (with or without other people around), in order to embarress that person, bring them down or prevent/break the peace.

Solution 1:

'Calumny' is close, though it's a legal term, and it refers to making statements wreck someone's reputation (which can't be done if it's done in private.) 'Slander' is probably better. 'Libel' is no good, as it happens in a publication.

Solution 2:

You can use rumormonger, rumor-monger, or rumor monger to describe such a person, with the assumption that rumors usually are false, or at least unverified.

a person given to spreading rumors, often maliciously.

rumor: a story or statement in general circulation without confirmation or certainty as to facts

If you want a more negative connotation, you can use slanderer.

slander: a malicious, false, and defamatory statement or report: a slander against his good name. Related forms: slan·der·er, noun