The dragon fiercely breathed fire at the knight.

Meaning: This means the dragon was releasing fire from its mouth in a wild and aggressive way, like it was trying to hurt the knight.

Choice of Words: Fiercely means in a wild, aggressive, or violent manner. Breathed fire means to release fire from the mouth, like a dragon. When you put them together, "fiercely breathed fire" means to release fire from the mouth in a wild, aggressive, or violent manner.