An Assassin can choose to deploy 'hidden' within another friendly Dark Elf infantry unit (but not Harpies) – make note of which unit is concealing the Assassin.

A hidden Assassin is not placed on the table during deployment, but is revealed later during the game. If the concealing unit is wiped out or flees from the battlefield before the Assassin is revealed, the Assassin counts as a casualty. There is no other way an Assassin can be harmed before he is revealed.

Hidden Assassins may be revealed at the start of any of your Movement phases, or at the start of either player’s Close Combat phase; declare that the unit in question contains an Assassin and place the model anywhere in the front rank of that unit, displacing the models as you normally would if a character had joined the unit.