The Calondon was a Kressari freighter that Odo secretly boarded in 2370, discovering that the Cardassians were using it to supply weapons to the Bajoran extremists of the Circle.
The Calondon appears in the Deep Space Nine episode The Circle, the middle part of the three-part story that opens the second season. With the Circle preparing a coup to expel the Federation from Bajor, Odo presses Quark for the source of the group's weapons, and Quark points him to the Kressari, botanical traders with no obvious interest in arms.
Odo slips aboard the Calondon in disguise as it leaves the station and stays hidden while it meets a Cardassian ship. He watches the Kressari captain take delivery of crates of weapons.
The evidence shows that the Circle is being armed, without knowing it, by the Cardassians, who expect to return to Bajor once the Federation has been forced out.