*Lewis - the Original Nerd Ambusher
Indirectly suggested by my good friend Jim Emerick, TO for the Midwest Rampage.
Ambushers - pg 79
A unit with the Ambushers rule does not deploy at the start of the battle. Instead, from Turn 2 onwards, the controlling player rolls a dice at the start of the turn for each unit of his Ambushers that have yet to arrive. On a 1 or 2, the Ambushers have been delayed - but you'll be able to 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 determing the victor.
Arriving Ambushers enter the battlefield from any point on any battlefield edge, and move on using the rules for reinforcements (see page 27).
*Note - Beastmen have their own special version of ambush in their book
This will apply though to any of the TK models with the "Entombed Beneath the Sands" special rule.
The big change for them from the previous book is that Ambushing is no longer optional.