When you deal the finishing blow to a creature you can predict the last words that creature will say. This may to be cast after the damage is rolled, but before the output is determined. You have to be able to understand the creature's language, however the creature does not need to understand you.

The caster determines what the creature's last words are, but they have to be something the creature might realistically say. Your DM may decide that the last words chosen are inappropriate and ask you to choose again.

If the creature is merely knocked unconscious you gain advantage on attacks that deal psychic damage on the creature for the next day