There are those troops who specialise in outflanking the foe, appearing from an unexpected quarter to wreak maximum damage.
Before starting deployment, a unit with the Ambushers special rule can choose to not deploy at the start of the battle. Instead, from Turn 2 onwards, the controlling player rolls a dice at the start of their turn for each unit of their Ambushers that have yet to arrive. On a 1 or 2, the Ambushers have been delayed - roll for them again next turn. On a 3 or more, the unit of Ambushers arrives, and will enter the board during the Remaining Moves sub-phase. If the Ambushers do not turn up for the entire game, they are assumed to have got lost, and are treated as having fled the battle for the purposes of determining the victor.
Arriving Ambushers enter the battlefield from any point on any battlefield edge, and move on using the rules for reinforcements. Note that a character may only join a unit deploying with the Ambushers special rule if they also have the same rule.
In addition, for every Core unit that deploys as Ambushers in your army, you are required to include at least one other Core Unit that does not deploy using the Ambushers rule (for more information, see the Choosing Your Army chapter).