Warp Spiders

Refer to your Codex for details.

This unit is eligible to shoot in a turn in which it Advanced, but if it does so, then until the end of the phase, models in this unit can only make attacks with Assault or Pistol weapons they are equipped with, and when resolving those attacks, this unit is treated as having Remained Stationary.
In your Shooting phase, after this unit has finished making its attacks, unless it Fell Back or Advanced this turn, this unit can make a Battle Focus move. When it does so, roll one D6: each model in this unit can make a Normal Move of up to the distance in inches equal to the result, as if it were your Movement phase. A unit that makes a Battle Focus move cannot embark within a TRANSPORT model at the end of that move, and until the end of the turn, such a unit is not eligible to declare a charge. A unit cannot make a Battle Focus move if it arrived as Reinforcements this turn, and a unit cannot make more than one Battle Focus move per turn.
NOTE: Battle Focus Moves Through Area Terrain
If a unit makes a Battle Focus move and any of its models wish to move over any part of an Area Terrain feature, subtract 3" from the distance models in that unit can move when making that Battle Focus move (to a minimum of 0). This penalty applies even if every part of that Area Terrain feature is 1" or less in height, but is not cumulative with any other penalties that Area Terrain feature may impose (e.g. the Difficult Ground terrain trait).

During deployment, if every model in this unit has this ability, you can set up this unit preparing a sudden assault instead of setting it up on the battlefield. If you do so, then in the Reinforcements step of one of your Movement phases you can set up this unit anywhere on the battlefield that is more than 9" away from any enemy models.