Seldom will a Clan Eshin Master Assassin reveal their presence unless a hidden army of Gutter Runners lurks close by.
When a friendly unit of Gutter Runners with the Ambushers special rule arrives from reserve, it can be placed on the battlefield anywhere completely within 12" of a revealed Master Assassin, but not within 6" of any enemy models (rather than entering the battle as reinforcements). The unit cannot charge during this turn and counts as having moved for the purposes of shooting, but can otherwise act as normal.
Note that this character cannot use this special rule whilst it remains hidden.