While on a mission to the Tzenkethi border, the crew of the USS Defiant discovers a Changeling infiltrator has sabotaged the ship to spark an unprovoked war between the Federation and the Tzenkethi. Major Plot Points

Before dying, the Changeling whispers a haunting warning to Odo: "You are too late. We are everywhere." DS9, Episode 3x26, The Adversary : r/StarTrekViewingParty

In a desperate struggle in main engineering, Odo kills the infiltrator. This makes Odo the first Changeling ever to harm one of his own , a violation of their species' core law that "no Changeling has ever harmed another".