The one where you find those responsible for the destruction of Romulus etc. Reading up about it on the forum I found this...
Originally Posted by SteelPaladin
A group of Remans caused the supernova/subspace explosion at the direction of Taris (who was later declared Romulan Praetor after the explosion). Taris herself was just following orders from what she refers to as her "dread masters." You briefly encounter a ship belonging to these "masters" over Iconia, and it is implied via the setting (and the name of an upcoming STF) that they are Iconians.
I didn't encounter any such ship. Though one of my Science officers did give me a message screen before beaming down to the planet that implied I had engaged some ship which didn't cloak, but seemed to be transported away. Unfortunately I never saw or interacted with any such ship.