Black Steed
The Black Riders#74 (x3/x1)
Treachery

When Revealed: Attach to a Nazgûl enemy in play, if able. (Counts as a Mount attachment with the text: "Limit 1 per enemy. Attached enemy gets -10 engagement cost and gains: 'Forced: At the end of the refresh phase, return attached enemy to the staging area.'")